The Boeing B-9 was the first all-metal monoplane bomber aircraft designed for the United States Army Air Corps. The first service model, dubbed the YB-9, was originally tested and developed by the United States aircraft manufacturing company Boeing as XB-901 and first flew on April 29, 1931. The YB-9 was an enlarged alteration of the Boeing Model 200 Commercial Transport. The Pratt & Whitney R-1830-13 radial engines used on the YB-9 gave it a top speed of 163 mph (262 km/h).
The second test model, named the Y1B-9 (Y1B- indicating funding outside normal fiscal year procurement), used liquid-cooled Curtiss V-1570-29 Conqueror engines. The increased power from these engines, combined with increased streamlining of the engine nacelles, increased its top speed to 173 mph (278 km/h). With the exception of the B-2 Condor, liquid-cooled engines were never used on production bombers for the United States military. The air-cooled radial engine was lighter and more reliable than the liquid-cooled engine, and less vulnerable to enemy damage.
The Y1B-9 was an improved version of the YB-9, featuring more powerful engines and a redesigned vertical stabilizer. Utilising two Pratt & Whitney R-1860-11 Hornet engines, the plane was faster than any contemporary fighter aircraft. The Y1B-9 was the first enclosed-cockpit plane flown by the Army. Its high speeds made open cockpits extremely impractical. Five were ordered by the Army in September 1931.