The Martin B-12 was a production version of the B-10 that was powered by the Pratt & Whitney Hornet radial instead of the Wright Cyclone. It was common in those days to assign different USAAC model numbers to aircraft of a given type which differed from each other only in the type of engines which powered them.
On January 17, 1933, the Army had ordered 48 production examples of the Martin Model 139 monoplane bomber design. 32 of the aircraft on this original order were ordered as YB-12s and B-12As, 7 were YB-12s and 25 were B-12As. They differed from the YB-10 primarily in being powered by Pratt and Whitney R-1690-11 Hornet radials in place of the Wright Cyclones of the B-10 series.
The first YB-12 appeared in February of 1934. The additional power provided by the Hornet engines gave the YB-12 a speed advantange of about 5 mph over the B-10B. Despite their new model number, the B-12s were otherwise quite similar to the B-10. They could be externally distinguished from the B-10 version by the presence of oil cooler intakes on the port side of the engine nacelles. Internally, the B-12A had provision for an extra fuel tank in the bomb bay. This tank had a capacity of 265 US gallons, supplementing the 226 US gallons normal fuel capacity on long flights.
In January of 1931, the US Army was assigned the responsibility for coastal defense around the United States mainland. As part of this mission, several Army B-12As were fitted with large floats for water-based operations. A B-12A fitted with twin floats set a seaplane speed record on August 24, 1935.
The B-12s remained in service with Army bombardment squadrons until the advent of the B-17 and B-18 in the late 1930s. The advances in bomber technology suddenly became so rapid that the B-10/B-12 series of bombers, revolutionary as they were at the time of their appearance, swiftly became obsolete as the 1930s progressed. By 1940, the B-10s and B-12s were thoroughly out of date and had been largely relegated to secondary roles such as target towing. No US Army B-10Bs or B-12s participated in any combat during World War 2.
Serials of B-12:
33-155/161 Martin YB-12 33-163/177 Martin B-12A 33-258/267 Martin B-12A
Specification of Martin YB-12:
Two Pratt & Whitney R-1690-11 Hornet air-cooled radial engines, rated at 700 hp at 6500 feet. Maximum speed 212 mph at 6500 feet, 190 mph at sea level. Initial climb rate 1740 feet per minute. An altitude of 10,000 feet could be attained in 10.1 minutes. Cruising speed 170 mph. Landing speed 71 mph. Service ceiling 24,600 feet. Absolute ceiling 26,600 feet. Normal range 524 miles, maximum range 1360 miles, Weights: 7728 pounds empty, 12,824 pounds gross Dimensions: wingspan 70 feet 6 inches, length 44 feet 9 inches, height 15 feet 5 inches, wing area 678 square feet. One 0.30-inch Browning machine gun in nose turret, one 0.30-inch Browning machine gun in flexible mount in dorsal gunner position, and one 0.30-inch Browning machine gun in a ventral tunnel. 2260 pounds of bombs could be carried.