Originally ordered by the United States Army Air Corps as the LB-13 light bomber.

The first B-3A (S/N 30-281) was converted to Y1B-4 configuration with the addition of R-1860-7 radial engines and low pressure tires.

On April 28, 1931, the army ordered 25 improved Y1B-4s as the Keystone B-4A.

This production version was part of the last biplane bomber order made by the Army Air Corps (along with 39 B-6As, identical in all respects except their make of engine), and the B-4As, delivered between January and April 1932, were the last biplane bombers delivered to the Air Corps.

The B-4 was the last of the Keystone biplane bombers ordered by the U.S. Army in late 1931.