The 17th production Keystone LB-6 on the 1929 contract (29-027) was completed as the LB-10. The LB-10 differed from the LB-6 in being powered by a pair of experimental 525 hp Wright R-1750-1 Cyclone radial engines, plus it had a single rudder in place of the twin rudders which Keystone had standardized on the LB-5A.
The single-rudder adaptation introduced by the LB-10 impressed the USAAC, and 63 examples were ordered in 1930 under the designation LB-10A. The LB-10A differed from the LB-10 in being powered by a pair of Pratt & Whitney R-1690-3 radials. The LB-10A had a slightly smaller wingspan and had a slightly shorter fuselage, but was otherwise similar to the LB-10. However, before the first LB-10A could be delivered, the USAAC had dropped the LB designation and was listing all of its bombers under the B series. The LB-10A was redesignated B-3A under the new scheme.
Specification of the Keystone LB-10:
Two 525 hp Wright R-1750-1 Cyclone alr-cooled radial engines. Maximum speed 116 mph at sea level, 113 mph at 5000 feet. Cruising speed 93 mph. Landing speed 58 mph. Service ceiling 13,440 feet. Absolute ceiling 15,800 feet. Initial climb rate 660 feet per minute. An altitude of 5000 feet could be attained in 9 minutes. Range was 350 miles with 2587 pounds of bombs. Weight: 6993 pounds empty, 13,285 pounds gross. Wingspan 75 feet, length 49 feet 3 inches, height 15 feet 6 inches, wing area 1148 square feet. Two Lewis machine guns in an open gunner's position in the nose, two Lewis machine guns in an open dorsal gunner's position, one Lewis gun firing downward through an opening in the lower fuselage.