Keystone Aircraft Corp LB-13 / Y1B-4 / B-4 / B-6
Developed from the Keystone B-3, the Keystone B-4 was a biplane bomber produced for the United States Army Air Corps in 1930. Originally seven were ordered by the United States Army Air Corps as the LB-13 light bomber. They were to be equipped with single vertical tails and were to be powered by a pair of 525 hp Pratt & Whitney GR-1690 radials. Serials were 30-344/353. When the LB- designation was dropped in 1930, the first five planes were redesignated Y1B-4. (The Y1B- designation indicates that funds for the design did not come from the normal annual funds.) Of seven LB-13s ordered, five were completed as Y1B-4s with 575 hp R-1860-7 engines (30-344/348). The Y1B-4 had a slightly better performance than the B-3A because of the more powerful engines, but was otherwise almost exactly the same.
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. Because of more powerful engines, the performance of the Y1B-4 was a slight improvement on the B-3, but the only difference between the two planes was their engines.
On April 28, 1931, the army ordered 25 improved Y1B-4s as the Keystone B-4A. Serials were 32-117/141. 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.
Like the B-3A, the B-4A carried five crew members; two pilots, a bombardier, and a front and rear gunner. The B-4A was externally almost identical to the B-3A which preceded it (as well as to the B-5 and B-6 which followed it).
B-4 was the last of the Keystone biplane bombers ordered by the U.S. Army in late 1931. These aircraft were used primarily as observation and reconnaissance aircraft as early as 1934 when the Martin B-10B went into operational service. Some remained in service into the early 1940s.
In 1932, Keystone produced 25 B-4As (575-hp R-1860-7 Hor-nets) and 39 B-6As (575-hp R-1820-1 Cyc-lones). Like the B-3A, these also had single tails; their armament was reduced to three 0.30-in (7.62-mm) guns, and bombload increased to 1130 kg (2500 lb).