The Martin B-10 was the first all-metal monoplane bomber to enter full production for the US Army. It was also the first bomber to have a performance that exceeded that of contemporary pursuit aircraft.
The immediate ancestor of the B-10 was the Martin Model 123, which was designed and built as a private venture by the Glenn L. Martin Company of Baltimore, Maryland. It was a midwing all-metal monoplane. The monocoque fuselage had corrugated top and bottom surfaces with a deep belly. The deep belly carried doors for an internal bomb bay, so the bombs could be carried internally rather than on external racks. The main landing gear retracted backwards into the rear of the engine nacelles, but the lower half of the wheels remained exposed. Four crew members were to be carried. Three of the crew members were seated in separate open cockpits on the top of the fuselage. The nose gunner/bombardier had a transparent aiming position in the lower nose, the pilot sat in an open cockpit abreast of the forward wing, and the rear gunner sat in an open position in the rear dorsal fuselage. The fourth crew member occupied a position inside the fuselage.
The Model 123 flew for the first time at Baltimore on February 16, 1932. It was powered by a pair of 600 hp Wright SR-1820-E Cyclone engines that were enclosed by NACA low-drag cowling rings. The wingspan was 62 feet 2 inches. The Model 123 was delivered to the Army on March 20, 1932. Trials began in July, 1932, during which a maximum speed of 197 mph was recorded at an altitude of 6000 feet. This was a truly spectacular performance for 1932. As a result of these trials, the open-cockpit gun position in the nose was replaced by a transparent, manually-rotated gun turret equipped with a single 0.30-inch machine gun. The pilot's cockpit and the dorsal gunner position remained open. At the same time, more powerful 675 hp R-1820-19 Cyclone engines were installed. These engines were also fitted with full cowlings that extended forward of the wings. A new longer-span wing was fitted, increasing the wingspan to 70 feet 7 inches. Despite an increase of nearly 2000 pounds in the gross weight to 12,230 pounds, the modified aircraft had a maximum speed of 207 mph at 6000 feet making it was faster than any US fighter then in service. On January 17, 1933, the Army ordered 48 production examples of the Martin design under the designation B-10.
The first 14 aircraft were designated YB-10. They were powered by 675 hp Wright R-1820-25 engines. They differed from the prototype primarily in having transparent sliding canopies fitted over both the pilot's cockpit and the rear gunner's position, a concession to the 200 mph-plus speeds that could be attained. The rear cockpit was modified to accommodate a radio operator in addition to the gunner. Armament consisted of a 0.30-inch Browning machine gun in the nose turret, a 0.30-inch gun in a flexible position in the dorsal position, plus a 0.30-inch machine gun in a tunnel position in the fuselage floor behind the bomb bay to guard against attacks from below. The internal bomb bay could carry two 1130-pound bombs or five 300-pound bombs. There were provisions for an external shackle under the right wing for a single 2000 pound bomb. The first YB-10 was delivered to Wright Field in November, 1933. Most of the YB-10s were based at March Field in California with the 7th Bomb Group until December, 1934, when it re-equipped with B-12s. The YB-10s then remained at March Field with the 19th Bomb Group. A turbocharged version of the YB-10 was designated the YB-10A but this proved too unreliable for service.
Production of the Martin bomber was continued by FY 1934 and 1935 Army procurements for a total of 103 examples of the B-10B, the primary service version. The B-10B could be distinguished from the YB-10 by the presence of air intakes on top the nacelle as well as by the relocation of the exhaust pipes from the lower nacelle to outlets at the nacelle top immediately behind the air intakes. It was otherwise quite similar to the service test YB-10. The first B-10B arrived at Wright Field in July 1935. Production deliveries to Langley Field began in December 1935 and were completed by August 1936. The B-10B served with the 2nd Bomb Group at Langley and the 9th at Mitchell Field. The B-10B served (along with YB-10s) with the 19th Bomb Group based at March Field in California. The B-10B also served with the 6th Bomb Group based in the Canal Zone, and was issued to the 28th Bomb Group based in the Philippines.
In January of 1931, the US Army was assigned the responsibility for coastal defense around the United States mainland. The B-10s 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, revolutionary though it was, swiftly became obsolete as the 1930s progressed. By 1940, the B-10B was thoroughly out of date and had been largely relegated to secondary roles such as target towing. No US Army B-10Bs participated in any combat during World War 2.
Specification of Martin B-10B:
Two Wright R-1820-33 Cyclone air-cooled radial engines, rated at 775 hp for takeoff and 750 hp at 5400 feet. Maximum speed 213 mph at 10,000 feet, 196 mph at sea level. An altitude of 5000 feet could be attained in 3.4 minutes. Cruising speed 193 mph. Landing speed 65 mph. Service ceiling 14,200 feet. Normal range 590 miles, maximum range 1240 miles, ferry range 1830 miles. Weights: 9681 pounds empty, 14,600 pounds gross, 16,400 pounds maximum. 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 position mounted in the floor of the fuselage behind the bomb bay. 2260 pounds of bombs could be carried.
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. The first YB-12 appeared in February of 1934. Despite their new model number, they were otherwise quite similar to the YB-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.
Specification of Martin B-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 ventral tunnel. 2260 pounds of bombs could be carried.
The Martin B-13 was a proposed version of the YB-10 powered by a pair of 650 hp Pratt & Whitney R-1860-17 engines. Twelve examples were ordered but were all cancelled before delivery.
The Martin XB-14 was a version of the YB-10 powered by a pair of 950hp Pratt & Whitney YR-1830-9 Twin Wasps. Only one example of this version was built.