What was Boeing’s Model 40, the company’s first mailplane, powered by?

Boeing’s Model 40, the company’s first mailplane, was powered by a single engine.

The Boeing Model 40 was developed in the 1920s for U.S. airmail routes. The original Model 40 used a water-cooled Liberty engine, while the substantially improved Model 40A used a single air-cooled Pratt & Whitney Wasp radial engine.

The Model 40A became especially important after Boeing Air Transport won a contract to carry U.S. mail between Chicago and San Francisco. Its operational success helped Boeing expand from aircraft manufacturing into airline operations and laid groundwork for the later United Air Lines organization.

The aircraft’s single-engine arrangement reflected the technology and economics of the period, when reducing weight and operating cost was crucial. It was much smaller and less powerful than later Boeing transports such as the four-engine 247 and 307 Stratoliner.