6 April 1938:¹ After being shipped by truck from the Bell Aircraft Company factory at Buffalo, New York, the XP-39 prototype, 38-326, made its first flight at Wright Field, Ohio, with test pilot James Taylor ² in the cockpit. During the test flight, Taylor flew the XP-39 to 390 miles per hour (628 kilometers per hour) at 20,000 feet (6,096 meters). The service ceiling was 32,000 feet (9,754 meters).
The Bell XP-39 Airacobra was a single-place, single-engine prototype fighter with a low wing and retractable tricycle landing gears. The airplane was primarily built of aluminum, though control surfaces were fabric covered.
As originally built, the XP-39 was 28 feet, 8 inches (8.738 meters) long with a wingspan of 35 feet, 10 inches (10.922 meters). The prototype had an empty weight of 3,995 pounds (1,812 kilograms) and gross weight of 5,550 pounds (2,517 kilograms).
The XP-39 was unarmed, but it had been designed around the American Armament Corporation T9 37 mm autocannon, later designated Gun, Automatic, 37 mm, M4 (Aircraft).³ The cannon and ammunition were in the forward fuselage, above the engine driveshaft. The gun fired through the reduction gear box and propeller hub.
The XP-39 was originally powered by a liquid-cooled, turbosupercharged and supercharged 1,710.597-cubic-inch-displacement (28.032 liter) Allison Engineering Co. V-1710-E2 (V-1710-17), a single overhead cam (SOHC) 60° V-12 engine with a compression ratio of 6.65:1. The V-1710-17 had a Maximum Continuous Power rating of 1,000 horsepower at 2,600 r.p.m. at 25,000 feet (7,620 meters), and Takeoff/Military Power rating of 1,150 horsepower at 3,000 r.p.m. at 25,000 feet, burning 91 octane gasoline.
The engine was installed in an unusual configuration behind the cockpit, with a two-piece drive shaft passing under the cockpit and turning the three-bladed Curtiss Electric constant-speed propeller through a remotely-mounted 1.8:1 gear reduction gear box. The V-1710-17 was 16 feet, 1.79 inches (4.922 meters) long, including the drive shaft and remote gear box. It was 2 feet, 11.45 inches (0.900 meters) high, 2 feet, 5.28 inches (0.744 meters) wide and weighed 1,350 pounds (612 kilograms).
After initial flight testing, the XP-39 was sent to the National Advisory Committee for Aeronautics (NACA) Langley memorial Aeronautical Laboratory at Hampton, Virginia. The prototype was tested in the Full-Scale Wind Tunnel. Improvements in aerodynamics were recommended and Bell rebuilt the airplane as the XP-39B with an Allison V-1710-E5 (V-1710-37) engine.
The turbosupercharger had been removed, which reduced the airplane’s power at altitudes above 15,000 feet (4,572 meters). The V-1710-37 had a maximum power of 1,090 horsepower at 3,000 r.p.m. at 13,300 feet (4,054 meters). This resulted in the P-39 being used primarily as a ground-attack weapon.
The XP-39B, with test pilot George Price in the cockpit, was damaged when when its landing gear did not fully extend, 6 January 1940. It was repaired and test flights resumed. On 6 August 1940, Captain Ernest K. Warburton stalled the prototype on landing. The impact resulted in significant structural damage, beyond economic repair. The airplane was later scrapped.
9,584 Bell P-39 Airacobras were built during World War II. More than half were sent to the Soviet Union.
¹ Reliable sources indicate the date of the first flight as both 1938 and 1939. The Bell Helicopter Company web site, “The History of Bell Helicopter: 1935–1949” states 1938.
² James Taylor may have been Lieutenant Commander James Blackstone Taylor, Jr., (1897–1942), Naval Aviator No. 437, a well-known U.S. Navy test pilot.
³ The 37-mm Aircraft Gun Matériel M4 is a recoil-operated aircraft weapon designed by John M. Browning. It has an overall length of 7 feet, 5 inches (2.26 meters). The barrel, or “tube,” is 5 feet, 5 inches (1.65 meters) long with a caliber of 1.457 inches (37.0 millimeters) and weighs 55 pounds (25 kilograms). The barrel is part of the recoiling section of the gun and moves rearward 9-5/8 inches (245 millimeters). The weight of the gun with a loaded 30-round magazine is 306.4 pounds (138.98 kilograms). The M4 fires a high-explosive tracer round with a muzzle velocity of 2,000 feet per second (607 meters per second). Each M54 shell is 9.75 inches (248 millimeters) long and weighs 1.93 pounds, of which the projectile makes up 1.34 pounds (0.608 kilograms). The cannon has a cyclic rate of fire of 125–150 rounds per minute.
Benjamin Scovill Kelsey, an aeronautical engineer and test pilot, despite his youth and junior rank, was one of the most influential Air Corps officers in the shaping of United States military air power during the years leading up to World War II.
Ben Kelsey was born in the Kelsey family home at 22 Johnson Street, Waterbury, Connecticut, on 9 February 1906. He was the son of Benjamin Richard Kelsey, an agent for the Waterbury Blank Book Co., and Elizabeth Anna Scovill Kelsey. His mother’s family, the Scovills, were a prominent manufacturing family in Connecticut. Kelsey’s father died 2 Dec 1909.
Kelsey attended Crosby High School in Waterbury. He had an early interest in aviation, and he began flight lessons at the age of 14. He enrolled with the Curtiss Flying School, located at Roosevelt Field, Long Island, New York. The school offered 500 minutes (8 hours, 20 minutes) of flight instruction for $500. Kesley soloed 13 August 1921.
Kelsey was able to buy a Curtiss “Jenny” airframe from the school, and accumulated parts and an OX-5 engine to put it into flying condition.
Kelsey was granted an aviator’s certificate, No. 6843, by the National Aeronautic Association of U.S.A. (N.A.A.), on behalf of the Fédération Aéronautique Internationale, about 1926. When the United States Department of Commerce began issuing pilot licenses after 1928, Kelsey qualified for Transport License No. 3200. He also held Department of Commerce Mechanic’s License No. 1368. (These, as well as a number of other licenses and identification cards belonging to Kelsey are held in the collection of the Smithsonian Institution National Air and Space Museum.)
Ben Kelsey entered the Massachusetts Institute of Technology (M.I.T.) at Cambridge, Massachusetts, in 1924, with a major in mechanical engineering. He was a member of the Reserve Officers Training Corps (R.O.T.C.), the Tau Beta Pi (ΤΒΠ) fraternity, and The Scabbard and Blade, a national military fraternity. He was president of the Aeronautical Engineering Society, a member of the Mechanical Engineering Society and the Society of Automotive Engineers (S.A.E.). Outside of academics, Kelsey participated in the varsity team sports of swimming and crew, and was a member of the Varsity Club and the Tech Boat Club. He also joined the M.I.T. Flying Club.
Kelsey graduated from M.I.T. in 1928 with a scientiae baccalaureus (S.B.) degree in Mechanical Engineering. He stayed on at the university as a Research Assistant to William G. Brown, Assistant Professor of Aeronautics, in the Department of Aeronautical Engineering.
Benjamin Scovill Kelsey was commissioned as a second lieutenant, Air Corps, United States Army, on 2 May 1929 (accepted 16 June 1929). Because of his experience as a pilot and his training in aeronautical engineering, Lieutenant Kelsey was assigned to work with First Lieutenant James Harold (“Jimmy”) Doolittle at the Guggenheim Full Flight Laboratory at Mitchel Field, Long Island, New York. Funded by the Daniel Guggenheim Fund for the Promotion of Aeronautics, Lieutenants Doolittle and Kelsey worked with Elmer Sperry, Jr., and Paul Kollsman to develop instruments and techniques to take off, fly, and land airplanes in clouds and fog.
On 24 September 1929, Lieutenant Kelsey, in the forward cockpit of a civil-registered, two-place, Consolidated NY-2 Husky biplane, NX7918, acted as a safety pilot while Doolittle flew the airplane from the rear cockpit. Doolittle had his visual reference to earth and sky completely cut off by a hood enclosure over his cockpit. He made the first completely blind airplane takeoff flight and landing, solely by reference to instruments on board his aircraft.
Lieutenant Kelsey was then assigned to the Air Corps Primary Flying School at Brooks Field, and the Advanced Flying School at Kelly Field, both located in San Antonio, Texas. He completed the Air Corps Pursuit Course in 1930.
On completion of his military flight training, Lieutenant Kelsey was assigned to the 77th Pursuit Squadron, 20th Pursuit Group, based at Mather Field, Sacramento, California, and then Barksdale Field in Louisiana. The squadron flew the Boeing P-12 biplane pursuit.
Kelsey was still a very junior officer in the Air Corps. In July 1931, he was number 231 on the list of second lieutenants.
Lieutenant Kelsey returned to M.I.T. to complete a master’s degree in aeronautical engineering. He was awarded a scientiae magister (S.M.) degree in Aeronautics in 1932.
The United States government had contracted with commercial aviation businesses to deliver the U.S. Mail by air. A corrupt contracting process was used by the Postal Service, however, and President Franklin D. Roosevelt cancelled those contracts and ordered the Air Corps to start delivering the mail, effective 19 February 1934.
The Air Corps organized the Army Air Corps Mail Operation. The Western Region, based at Salt Lake City, Utah, was under the command of Lieutenant Colonel Henry H. (“Hap”) Arnold (later, General of the Air Force). 2nd Lieutenant Kelsey was transferred from the navigator’s school at Rockwell Field, San Diego, California, to the A.A.C.M.O. Western Region. He and other Air Corps pilots flew at night, in winter weather, on a triangular route from Salt Lake City to Reno and Elko, Nevada. Initially, the airplanes used were the Boeing P-12 pursuits, but they were able to carry only a small amount of mail. The Curtiss A-12 Shrike was then used for the mail delivery.
During this time, Lieutenant Kelsey also worked with Captain Albert F. Hegenberger on an instrument flight research program that allowed Air Corps pilots to conduct a flight from start up to shut down, including takeoffs and landings in “zero-zero” conditions, solely by reference to instruments.
Kelsey was promoted to the rank of first lieutenant, 1 October 1934. He was next assigned as the Pursuit Projects Officer, Material Division, at Wright Field, Dayton, Ohio.
1st Lieutenant Benjamin S. Kelsey married Miss Caryl Rathje, 16 August 1935. Miss Rathje was the fourth of seven children of William John Rathje, president of the Mid-City Trust & Savings Bank in Chicago. Her mother was Mary Eliza Philpot Rathje.
Caryl had graduated from Morgan Park High School in 1929 and then attended Northwestern University at Evanston, Illinois. She was a member of the Beta Chapter Alpha Phi (ΑΦ) sorority. While at Northwestern, Miss Rathje sang 1st soprano in the Glee Club and was secretary of the Daughters of Neptune swimming team.
The wedding, performed by Rev. Clyde Melsee, took place in the bride’s home in Chicago, Illinois.
The Kelseys would have three children, Benjamin Scovill Kelsey, Jr., Peter Rathje Kelsey, and David W. Kelsey.
Kelsey continued his assignment at Wright Field: “Engineer, Procurement Branch. Project Officer, Pursuit, Engineering Procurement Branch, Material Division.” It was his task to evaluate all pursuit aircraft in current service with the Air Corps, and to assess its future needs. He was in contact aircraft and engine manufacturers, and evaluated similar aircraft types in foreign military service. He performed as an aeronautical engineer, a test pilot and a forecaster of aviation technology.
On Christmas Eve, 1936, Lieutenant Kelsey was flying a twin-engine Martin B-10 bomber from Mitchel Field, New York, to Wright Field. The flight was not uneventful.
Lieut. Ben Kelsey, returning to Wright Field one night in a Martin B-10B, had reached the vicinity of Columbus, Ohio, when the left engine quit “as though the switches were cut.” After some gas valve and ignition switch manipulating, the engine started up with the gas valve in the “off” position, gasoline flowing freely from the exhaust pipe. On starting, the flames ignited the gasoline on the wing, which also ignited the fabric rear portion of the wing. The engine then began to operate normally and, after deciding against taking a chance on the Columbia Airport fire fighting facilities, Lieut. Kelsey raced the fire to Wright Field. A few inches from the aileron the fire lost, and Lieut. Kelsey, after a few practice cloud landings, shot a “hot” flapless landing successfully at Wright Field with only a little more than half the left wing surface remaining. When asked why he didn’t jump, he answered: “I had an electric train in the back for the boy.”
—Air Corps NEWS LETTER, Vol. XX, No. 3, 1 February 1937, at Page 3, Column 1
For his actions that night, Lieutenant Kelsey was awarded the Distinguished Flying Cross.
The President of the United States of America, authorized by Act of Congress, July 2, 1926, takes pleasure in presenting the Distinguished Flying Cross to First Lieutenant (Air Corps) Benjamin S. Kelsey, U.S. Army Air Corps, for heroism and extraordinary achievement while participating in an aerial flight. On 24 December 1936, at about 7:30 p.m., Lieutenant Kelsey was piloting an airplane from Mitchel Field, Long Island, New York, to Wright Field, Dayton, Ohio, flying at an altitude of approximately 1,800 feet when he left motor instantaneously failed and the left wing burst into flames. By his outstanding courage, sound judgment, and at the risk of his life Lieutenant Kelsey maintained control of the airplane and, with great difficulty due to the darkness of the night and the glare from the burning airplane, effected a safe landing at Wright Field, thereby preventing the destruction to valuable Government property.
General Orders: War Department: American Decorations, 1937 – 1938 (Supplement II-1939)
Action Date: December 24, 1936
On 1 September 1937, Lieutenant Kelsey made the first flight of the Bell Aircraft Corporation XFM-1 Airacuda at Buffalo, New York. This was a prototype twin-engine heavy fighter. The Airacuda was Bell’s first military aircraft and was technologically advanced, but also had many flaws. Kelsey found that the airplane would immediately enter a spin when flown with one engine. It was also unstable in pitch, as the “pusher” arrangement would cause pitch angle to climb as engine power was increased. This was the same problem that had caused the Air Service to ground all pusher-type aircraft in February 1914, and to prohibit their future use.
The XFM-1 was powered by two Harold Caminez-designed, liquid-cooled, supercharged, 1,710.60-cubic-inch-displacement (28.032 liter) Allison Engineering Company V-1710-9 single overhead cam 60° V-12 engines. These were very early models of the type, but the Allison V-12 would be a significant feature in several future aircraft which were proposed by Kelsey.
In February 1937, the Air Corps issued a proposal to aircraft manufacturers, written by Lieutenant Kelsey and Lieutenant Gordon P. Saville, for a twin-engine, high-altitude interceptor. This would be developed into the Lockheed Aircraft Company’s legendary P-38 Lightning. Ben Kelsey made the first flight of the XP-38 at March Field, Riverside, California, 27 January 1939. The Lightning was the first production airplane to use the Allison V-1710 engine.
Attempting a transcontinental speed record to draw public attention to the Air Corps’ need for technologically advanced aircraft, Kelsey flew the prototype XP-38 from March Field to Mitchel Field. On approach to Mitchel, the XP-38 was behind several slower training planes, so Lieutenant Kelsey throttled back the engines. When he tried to throttle up, the carburetor venturis iced and the engines would not accelerate, remaining at idle. With insufficient power to maintain altitude, the airplane crashed on a golf course short of the airport.
The XP-38 was damaged beyond repair, but its performance on the transcontinental flight was so impressive that 13 YP-38A pre-production aircraft were ordered from Lockheed by the Air Corps. Testing continued with the YP-38A and was the new fighter was quickly placed in full production. The P-38 Lightning was one of the most successful combat aircraft of World War II. By the end of the war, Lockheed had built 10,037 Lightnings.
Another Air Corps proposal written by Lieutenants Kelsey and Saville resulted in the Bell XP-39 Airacobra. This aircraft was also powered by an Allison V-1710, placed in a position behind the cockpit. The XP-39 made its first flight 6 April 1938. 9,584 Bell P-39 Airacobras were built during World War II.
Curtiss-Wright’s Chief Engineer, Donovan Reese Berlin, had taken the tenth production P-36A Hawk, Air Corps serial number 38-10, and had its air-cooled radial engine replaced with a liquid-cooled Allison V-1710-C13 V-12 engine. Although the P-36A’s original Pratt & Whitney R-1830-17 14-cylinder radial engine had greater displacement and produced 80 horsepower more for takeoff than the Allison V-12, the long, narrow V-12 allowed for a much more streamlined engine cowling for higher speed and greater efficiency.
The XP-40 was disappointingly slow when compared to the P-36A, however. Kelsey had the prototype sent to the National Advisory Committee for Aeronautics (NACA) Research Center at Langley Field, Hampton, Virginia, where the full-size airplane was placed inside a wind tunnel.
Over a two-month period, NACA engineers made a number of improvements. The radiator was moved forward under the engine and the oil coolers utilized the same air scoop. The exhaust manifolds were improved as were the landing gear doors.
When they had finished, Lieutenant Kelsey flew the modified XP-40 back to Curtiss. Its speed had been increased to 354 miles per hour (570 kilometers per hour), a 12% improvement.
Between 1939 and 1945, Curtiss built 13,738 P-40s in many configurations. They flew in combat in every theater of operations during World War II.
1st Lieutenant Kelsey was promoted to the rank of captain in May 1939. In July 1940, Captain Kelsey was sent to England as a military attaché. He observed combat operations during the Battle of Britain, and was able to fly the Hawker Hurricane and Supermarine Spitfire fighters. Kelsey arranged to have two Spitfire Mk.Va fighters, R7347 and W3119, shipped to the United States for evaluation at Wright Field and NACA Langley.
Captain Kelsey returned to the United States, departing Galway, Ireland, 5 July 1940, aboard the United States Lines passenger liner S.S. Washington. The ship was under the command of Captain Harry Manning, who had been one of two navigators intended to fly with Amelia Earhardt around the world in 1937. Kelsey arrived at the Port of New York on 13 July. [Interestingly, on 11 June 1940, S.S. Washington had been stopped off the coast of Portugal by a Kriegsmarine U-boat, U-101, and the passengers and crew were given ten minutes to abandon ship before it was to be sunk. The submarine’s captain, Kapitänleutnant Fritz Frauenheim, realized that he had mistaken the ship for another, and signaled to Captain Manning, “Thought you were another ship. Please go on, go on.”]
With World War II underway, Britain could not produce enough combat aircraft to meet its immediate needs. The British Purchasing Commission had asked North American Aviation to produce Curtiss-Wright Tomahawks (P-40s) under license. N.A.A. responded with a proposal for an all-new fighter, the NA-73X, which would be powered by the Allison V-1710, could be designed and built in the same time it would take to establish a P-40 assembly plant in California. On 10 April 1940, the Commission authorized North American to proceed. The prototype made its first flight on 26 October 1940, and it was placed in production as the Mustang Mk.I.
The fourth and tenth production Mustang Mk.Is, AG348 and AG354, were diverted to the Army Air Corps, designated XP-51 and assigned serial numbers 41-038 and 41-039. Both aircraft were extensively flight tested at Wright Field and by NACA at the Langley Memorial Aeronautical Laboratory. The airplane was ordered into production as the P-51A Mustang.
Lee Atwood, North American’s chief engineer and later president, wrote:
Ben Kelsey, in my opinion, was among the most effective Air Corps officers of World War II. His active liaison between combat and aircraft engineering was extremely productive and resulted in aircraft and weapons improvements in a timely manner and when most critically needed. I first met Ben when he was the project officer on the P-38 and he became interested in the P-51 at an early stage. Undoubtedly, he did all he could to bring it along. He had a low key, but very convincing approach.
—Warbird Factory: North American Aviation in World War II, by John Fredrickson, Voyageur Press, 2015, Chapter 8 at Page 147
Captain Kelsey was promoted to the rank of major (temporary) 15 March 1941. On 10 October 1941, he was appointed a major in the wartime Army of the United States (A.U.S.).
As the United States prepared for an unavoidable involvement in World War II, its military forces underwent a massive expansion. Promotions for serving officers came much more rapidly than had been the case during the pre-war years. Normally, they would retain their permanent rank in the United States Army while rising to higher rank and responsibility in the Army of the United States.
Major Kelsey was promoted to lieutenant colonel (temporary) on 5 January 1942, and to lieutenant colonel, A.U.S., on 1 February 1942. Four weeks later, 1 March 1942, he was advanced to colonel, A.U.S.
Colonel Kelsey was involved in the planning and operation of the North Atlantic Ferry Route (Operation Bolero), by which aircraft were moved from the United States to Europe, by way of Newfoundland, Labrador, Greenland, Iceland and Ireland. He flew a Lockheed P-38F Lighting of the 14th Fighter Group on the first crossing by this route in June 1942.
Colonel Kelsey resumed his assignment as chief of the Pursuit Branch at Wright Field. On 9 April 1943, Kelsey was at Lockheed in southern California, testing a P-38G-10-LO Lightning, 42-12937. He had entered a high-speed dive at 35,000 feet (10,668 meters). When he attempted to engage the new “dive flap,” intended to control some of the compressibility effects that pilots had encountered, the control lever broke away. Kelsey applied full rudder and aileron. The P-38’s left wing and tail broke away, and the remaining part of the airplane entered an inverted spin. Kelsey bailed out, suffering a broken ankle. The airplane crashed near Calabasas, California (13.5 miles, or 21.8 kilometers, from where I am now writing).
Colonel Kelsey was named chief of the Flight Research Branch, Flight Test Division, at Wright Field, in July 1943.
In November 1943, Colonel Kelsey was assigned as deputy chief of staff, Headquarters, IX Fighter Command, based at Army Air Forces Station 449 (RAF Middle Wallop) in Hampshire, England. Shortly after, he was reassigned as chief of operations, Eighth Air Force. While in England, Colonel Kelsey flew 21 combat missions. During 1944, he flew combat missions is the P-51 Mustang with the 363d Fighter Squadron, 364th Fighter Group, which was based at RAF Honington (AAF Station 375) in Suffolk, England.
Following the end of the war, Colonel Kelsey returned to Wright Field. He reverted to his permanent rank of major, Army Air Forces, 2 May 1946. He then had a series of staff assignments. After the United States Air Force was established as a separate military service on 18 September 1947, Major Kelsey was appointed a major, U.S. Air Force, with a date of rank 2 May 1946. Major Kelsey was promoted to the rank of colonel U.S. Air Force, 2 April 1948.
Colonel Kelsey attended the National War College at Fort Lesley J. McNair, Washington, D.C., from August 1948 to June 1949. He then became an instructor at the College. Kelsey next served as Deputy Director for Research and Development at Air Force headquarters. He was promoted to the rank of brigadier general (temporary) on 5 September 1952.
In 1954, General Kelsey was the Air Force representative to the Research Airplane Committee which proposed the hypersonic X-15 rocketplane, which would be built by North American Aviation.
On 30 December 1955, Brigadier General Kelsey reverted to his permanent rank of colonel. He retired from the Air Force 31 December 1955, after 26 years, 7 months, 30 days of military service.
During his military career, Colonel Kelsey had been awarded the Distinguished Service Cross, the Legion of Merit, the Distinguished Flying Cross, and the Air Medal with two oak leaf clusters (three awards). France and Belgium each decorated him with their Croix de Guerre.
In 1944, Colonel Kelsey had been honored with the Octave Chanute Award of the Institute of Aeronautical Sciences “for an outstanding contribution made by a pilot or test personnel to the advancement of the art, science, and technology of aeronautics.”
In 1959, the Massachusetts Institute of Technology bestowed on Kelsey its Jerome C. Hunsaker Visiting Professor of Aerospace Systems. Professor Kelsey delivered the annual Minta Martin Lecture, “Size Considerations in Optimum Aircraft.”
The National Air and Space Museum selected Colonel Kelsey as the Charles A. Lindbergh Chair in Aerospace History for 1979. With this fellowship, he wrote The Dragon’s Teeth?: The Creation of United States Air Power in World War II (Smithsonian Institution Press, 1982).
Colonel Benjamin Scovill Kelsey, United States Air Force (Retired), died at Sherwood Farm, Stevensburg, Virginia 3 March 1981, at the age of 74 years. His remains were cremated.
This Day in Aviation is indebted to Ms. Tiffany Kelsey, Ben Kelsey’s granddaughter, for her invaluable assistance.
9 August 1939: After General Henry H. Arnold had ordered that the prototype Bell Aircraft Corporation XP-39 Airacobra be evaluated in the National Advisory Committee for Aeronautics (NACA) Full-Scale Tunnel at the Langley Memorial Aeronautics Laboratory, Langley Field, Virginia, it was flown there from Wright Field. It was hoped that aerodynamic improvements would allow the prototype to exceed 400 miles per hour (644 kilometers per hour).
NACA engineers placed the full-size airplane inside the large wind tunnel for testing. A number of specific areas for aerodynamic improvement were found. After those changes were made by Bell, the XP-39’s top speed had improved by 16%.
The Bell XP-39 Airacobra was a single-place, single-engine prototype fighter with a low wing and retractable tricycle landing gears. The airplane was primarily built of aluminum, though control surfaces were fabric covered.
As originally built, the XP-39 was 28 feet, 8 inches (8.738 meters) long with a wingspan of 35 feet, 10 inches (10.922 meters). The prototype had an empty weight of 3,995 pounds (1,812 kilograms) and gross weight of 5,550 pounds (2,517 kilograms). Changes recommended by NACA resulted in a recontoured canopy, lengthened the airplane to 29 feet, 9 inches (9.068 meters) and reduced the wing span to 34 feet, 0 inches (10.362 meters). Its empty weight increased to 4,530 pounds (2,055 kilograms) and gross weight to 5,834 pounds (2,646 kilograms).
The XP-39 was unarmed, but it had been designed around the American Armament Corporation T9 37 mm autocannon, later designated Gun, Automatic, 37 mm, M4 (Aircraft). The cannon and ammunition were in the forward fuselage, above the engine driveshaft. The gun fired through the reduction gear box and propeller hub.
The XP-39 was originally powered by a liquid-cooled, turbosupercharged and supercharged 1,710.597-cubic-inch-displacement (28.032 liter) Allison Engineering Co. V-1710-E2 (V-1710-17), a single overhead cam (SOHC) 60° V-12 engine with a compression ratio of 6.65:1. The V-1710-17 had a Maximum Continuous Power rating of 1,000 horsepower at 2,600 r.p.m. at 25,000 feet (7,620 meters), and Takeoff/Military Power rating of 1,150 horsepower at 3,000 r.p.m. at 25,000 feet, burning 91 octane gasoline. The engine was installed in an unusual configuration behind the cockpit, with a two-piece drive shaft passing under the cockpit and turning the three-bladed Curtiss Electric constant-speed propeller through a remotely-mounted 1.8:1 gear reduction gear box. The V-1710-17 was 16 feet, 1.79 inches (4.922 meters) long, including the drive shaft and remote gear box. It was 2 feet, 11.45 inches (0.900 meters) high, 2 feet, 5.28 inches (0.744 meters) wide and weighed 1,350 pounds (612 kilograms).
Army Air Corps strategy changed the role of the P-39 from a high-altitude interceptor to a low-altitude tactical strike fighter. The original turbocharged V-1710-17 was replaced with a V-1710-37 (V-1710-E5) engine. The turbosupercharger had been removed, which reduced the airplane’s power at altitudes above 15,000 feet (4,572 meters). The V-1710-37 had a maximum power of 1,090 horsepower at 3,000 r.p.m. at 13,300 feet (4,054 meters). With the NACA-recommended aerodynamic changes and the new engine, the prototype Airacobra was redesignated XP-39B.