1 May 1937: The Lockheed Aircraft Company’s chief test pilot, Marshall Headle, took the Lockheed XC-35, Air Corps serial number 36-353, for its first flight. Ordered by the Air Corps in 1936 as a high-altitude research aircraft, and for the development of cabin pressurization, the XC-35 Supercharged Cabin Transport Airplane was a highly modified Lockheed Electra 10A. It was the first airplane to be specifically built with a pressurized cabin.

The Citizen-News, Hollywood, California, reported:

Mystery Airplane In Air For Tests

     The Army’s secretly built stratosphere plane took off on its first test flight today at the Lockheed Aircraft Corp’s factory at Burbank with Marshall Headle, chief test pilot for the company, its sole occupant.

     Only routine, low altitude maneuvers were to be carried out today, however, to test the plane’s flying characteristics, and it was expected that high altitude tests would not begin until next week.

     Shortly before noon a canvas curtain that was erected about the mystery plane immediately after an unauthorized photograph brought stormy protests from the War Dept., was lowered and the big twin engined, low wing craft rumbled to the end of the factory airport.

     A quick sprint and the silver bodied plane was in the air safely.

     Company officials continued their silence on performance expected of the military experimental ship.

—Citizen-News, Vol. 33, No. 27, 1 May 1937, Page 3, Column 1

The XC-35 had been rolled out of the plant several days earlier. It was photographed by a newspaper photographer and the image published in a local newspaper. The photographer had hidden behind bushes on a small hill near the plant. This resulted in an immediate response from the military, with the airplane being hidden behind a canvas enclosure. Armed guards were ordered to “shoot cameras out of the hands of photographers and ask questions afterward—but to destroy all cameras.” There were also roving vehicle patrols, etc. Army intelligence officers interrogated the photographer.

The Army Air Corps was awarded the Collier Trophy for 1937 for the XC-35 project.

With a strengthened circular fuselage and smaller windows, the XC-35′s passenger compartment was pressurized by engine turbo-superchargers and controlled by a flight engineer. Cabin pressure could be maintained at the equivalent of 12,000 feet (3,658 meters) above sea level, at an actual altitude of 30,000 feet (9,144 meters).

A crew of three and two passengers were accommodated within the pressurized section, and there was room for another passenger to the rear of the pressure bulkhead, which could only be used at lower altitudes.

The Lockheed XC-35 was similar to the Lockheed Model 10 Electra, with the same dimensions: length, 38 feet, 7 inches (11.760 meters), wingspan 55 feet (16.764 meters), and height, 10 feet, 1 inch (3.073 meters). It had a total wing area of 458.5 square feet (42.596 square meters). The airplane had an empty weight of 7,940 pounds (3,602 kilograms), and gross weight of 10,500 pounds (4,763 kilograms).

The Lockheed XC-35 was powered by two air-cooled, supercharged, 1,343.804-cubic-inch-displacement (22.021 liter) Pratt & Whitney R-1340-43 (Wasp T5H1) single-row, nine-cylinder radial engines with a compression ratio of 6:1. The R-1340-43 had a Normal and Takeoff Power rating of 550 horsepower at 2,200 r.p.m. from Sea Level to 3,000 feet (914 meters), burning 92-octane gasoline. It was direct drive. The engine was 3 feet, 6.25 inches (1.073 meters) long, 4 feet, 3.50 inches (1.308 meters) in diameter, and weighed 864 pounds (392 kilograms). The engines drove two-bladed, variable pitch propellers.

The XC-35 had a cruise speed of 214 miles per hour (344 kilometers per hour), and maximum speed of 236 miles per hour (380 kilometers per hour) at 20,000 feet (6,096 meters). It could climb at a rate of 1,125 feet per minute (5.7 meters per second), and its service ceiling was 31,500 feet (9,601 meters).

Able to fly above 30,000 feet (9,144 meters), the XC-35 was later used by NACA for thunderstorm penetration research flights. In 1948 it was transferred to the Smithsonian Institution.

© 2025, Bryan R. Swopes