The second test model, named the Y1B-9 (Y1 indicating funding outside normal fiscal year procurement), originally 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 Curtiss B-2 Condor, liquid-cooled engines were never used on production US military bombers, as air-cooled radial engines were lighter and more reliable than the liquid-cooled engines, and less vulnerable to enemy damage.

The Y1B-9A 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 equalled the speed of all existing American fighter aircraft. While enclosed canopys were considered, the B-9 was never fitted with them.

The B-9 was a truly revolutionary design, and had a speed fully 60 percent greater than that of the Keystone biplane bombers that were still the backbone of the American bomber force in 1932, but for all the advances the B-9 offered, it was quickly superseded by the performance of the next generation of Martin B-10 & Martin B-12 bombers.