Boeing Model 40

The Boeing Model 40 was a single-engine biplane mail plane of the 1920s, built after the United States Post Office asked in 1925 for a replacement for its ex-military DH-4 aircraft. The first Model 40 used the Liberty V12 engine, but the production order went to the Douglas M-2. Boeing revived the design as the Model 40A, with an air-cooled Pratt and Whitney Wasp radial engine and a small enclosed cabin for two passengers, and won the San Francisco to Chicago airmail contract in 1927 with a bid of three dollars per pound. Later versions carried four passengers and used Hornet engines. It was the first Boeing aircraft to carry passengers, and production continued until February 1932.