27 November 1933: The United States Army Air Corps accepted the Glenn L. Martin Company’s first service test YB-10 bomber, serial number 33-140. This was the first all-metal monoplane bomber with an internal bomb bay, retractable landing gear, rotating gun turret and enclosed cockpit. It flew faster than pursuit aircraft of the day.
There had been a single prototype, the Martin Model 123. It was powered by two Wright R-1820-19 engines rated at 600 horsepower, each. This was designated XB-907 by the U.S. Army Air Corps when tested at Wright Field in 1932. Recommendations for modifications were made, and Martin upgraded the prototype to the XB-907A configuration, which was then designated XB-10 by the Air Corps. The Army then ordered 48 production airplanes.
The first group of 14 airplanes were designated YB-10. The YB-10 (Martin Model 139) had enclosed canopies for the pilot and top gunner, and a nose turret. The crew consisted of a pilot, radio operator and three gunners.
These airplanes were powered by two air-cooled, supercharged, 1,823.129-cubic-inch-displacement (29.876 liter) Wright Cyclone SGR-1820-F2 (R-1820-25) 9-cylinder radial engines with a compression ratio of 6.4:1, which were rated at 750 horsepower at 1,950 r.p.m. at Sea Level. The engines turned three-bladed Hamilton Standard adjustable-pitch propellers through a 16:11 gear reduction. The R-1820-25 was 3 feet, 11–13/16 inches (1.214 meters) long, 4 feet, 5-¾ inches (1.365 meters) in diameter, and weighed 1,047 pounds (475 kilograms).
The bomber could carry two 1,130 pound (513 kilogram) bombs, or five 300 pound (136 kilogram) bombs in its internal bomb bay. Alternatively, a 2,000 pound (907 kilogram) bomb could be carried externally. There were three .30-caliber (7.62 mm) Browning M1919 machine guns for defense.
The first full scale production version was the B-10B, which was very similar to the service test YB-10s. These airplanes were 44 feet, 9 inches (13.640 meters) long with a wingspan of 70 feet, 6 inches (21.488 meters) and height of 15 feet, 5 inches (4.670 meters). The B-10B had an empty weight of 9,681 pounds (4,391 kilograms).
The engines installed in this variant were Wright Cyclone SGR-1820-F3 (R-1820-33), rated at 700 horsepower at 1,950 r.p.m. at Sea Level. Dimensions, weight and propeller gear reduction for this engine are the same as the R-1820-25, above.
The B-10B had a cruising speed of 193 miles per hour (311 kilometers per hour), and maximum speed of 213 miles per hour (343 kilometers per hour) at 10,000 feet (3,048 meters).
33-140 was converted to a B-10M for towing aerial targets and was assigned to the Tow Target Detachment at March Field, Riverside, California. Piloted by Robert E. Phillips, 33-140 was damaged in a taxiing accident, 8 April 1942.
© 2018, Bryan R. Swopesby