In 1930, seven Keystone biplane bombers were ordered under the designation LB-13. 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.
However, in the early 1930s, the USAAC abandoned its separate classification scheme for light (LB) and heavy (HB) bombers, and grouped them both under the B category. The bombers in the LB series already under order had to be redesignated. In particular, 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.
On April 28, 1931, the Army ordered 25 examples of the B-4A, which was an improved production version of the Y1B-4. Serials were 32-117/141.
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). These Keystone bombers usually differed
from each other only in the type of engine which powered them, and it
was often only possible to distinguish one from the other by an
examination of their serial numbers.
30-344/348 Keystone Y1B-4 32-117/141 Keystone B-4A
Specification of the Keystone B-4A:
Two 575 hp Pratt & Whitney R-1860-7 air-cooled radial engines. Maximum speed 121 mph at sea level, 103 mph cruising speed. Initial climb rate 690 feet per minute. An altitude of 5000 feet could be reached in 8.6 minutes. Service ceiling 14,000 feet. Range 855 miles. Weight: 7951 pounds empty, 13,209 pounds gross. Wingspan 74 feet 8 inches, length 48 feet 10 inches, height 15 feet 9 inches, wing area 1145 square feet. Armed with three Browning machine guns, one in each of nose, dorsal, and ventral positions. A bomb load of 2500 pounds could be carried.