The Boeing YB-9 was the first all-metal monoplane bomber aircraft designed for the United States Army Air Corps.

The Monomail was of radical design for the time, being a semi-monocoque, stressed skin cantilever monoplane with a retractable undercarriage.

Air Corps bomber squadrons of the day were largely equipped with slow strut-braced biplanes built from steel-tube frames covered in doped fabric, such as the Keystone B-6, and Boeing decided to design and build a twin-engined bomber using the same techniques used in the Monomail to re-equip the Air Corps.

[4][5] The first of the two prototypes to fly was the radial powered Model 215 which, carrying civil markings and the aircraft registration X-10633, made its first flight on 13 April 1931.

[2][8] The Y1B-9 (Y1 indicating funding outside normal fiscal year procurement), powered by two liquid-cooled Curtiss V-1570-29 'Conqueror' engines, first flew on 5 November 1931.