Lockheed XC-35

The Lockheed XC-35 is a twin-engine, experimental pressurized airplane, the second American aircraft to feature cabin pressurization. It was a development of the Model 10 Electra, designed to meet a 1935 request by the United States Army Air Corps for an aircraft to perform high-altitude research and test the feasibility of a pressurized cabin, at a total contract cost of 112,197 dollars. Air Corps structures experts Carl Greene and John Younger worked with Lockheed on a new circular-section fuselage able to withstand a differential of 10 pounds per square inch, with small windows to guard against a blowout. Cabin air was bled from the engines' turbo supercharger and could hold a cabin altitude of 12,000 feet at 30,000 feet. Two Pratt and Whitney XR-1340-43 engines of 550 horsepower each drove it. The XC-35 reached Wright Field in May 1937, made its first performance flight on 5 August, and its test program won the Army Air Corps the Collier Trophy. Its lessons shaped the Boeing 307 Stratoliner and the B-29 Superfortress. It was donated to the National Air and Space Museum in 1948.