It was the first to offer a pressurized cabin, allowing it to cruise at an altitude of 20,000 ft (6,000 m), well above many weather disturbances. The pressure differential was 2.5 psi (17 kPa), so at 14,700 ft (4,480 m) the cabin air pressure was equivalent to an altitude of 8,000 ft (2,440 m). The Model 307 had capacity for a crew of six and 33 passengers. The cabin was nearly 12 ft (3.6 m) across. It was the first land-based aircraft to include a flight engineer as a crew member (several flying boats had included a flight engineer position earlier). In addition to its civilian service it was also flown as the Boeing C-75 Stratoliner by the United States Army Air Forces, who used it as a long-range cargolift aircraft. In 1935, Boeing designed a four-engine airliner based on its B-17 heavy bomber (Boeing Model 299), then in development, calling it the Model 307. It combined the wings, tail, rudder, landing gear, and engines from their production B-17C with a new, circular cross-section fuselage of 138 in (351 cm) diameter, designed to allow pressurization. The first order, for two 307s (named Stratoliners), was placed in 1937 by Pan American Airways. Pan Am soon increased this to six, and a second order for five from Transcontinental & Western Air (TWA) prompted Boeing to begin production on an initial batch of the airliner. Pan Am received its initial order and TWA received its order, but only one of the second batch of four Pan Am aircraft was delivered before war intervened and put a halt to civil aircraft production. For information on the operational history, conversion to C-75, and the five variants, and the individual histories of the 10 airframes built, click here.
