Boeing B-9

The Boeing B-9, tested as the YB-9, Y1B-9 and Y1B-9A, was the first all-metal monoplane bomber designed for the United States Army Air Corps. It was a much enlarged twin-engine development of the single-engine Boeing Model 200 Monomail transport and used the same stressed-skin construction and retractable undercarriage. Boeing built the two prototypes as a private venture, one with liquid-cooled Curtiss Conqueror engines and one with Pratt and Whitney Hornet radial engines, and five service test aircraft followed. The first of them entered service with the 2nd Bomb Group in September 1932. Although it matched the speed of contemporary fighters, the Martin B-10 won the production order, so no serial production was started and the type was withdrawn in 1935.