<p>The Glenn L. Martin Company had been responsible for the supply of a fair proportion of the US Army Air Corps' bomber aircraft in the 1920s but, after being the unsuccessful contender in several design competitions, decided in 1930 to design and build an advanced bomber project as a private venture. Identified initially by the company as the Martin 123, this monoplane bomber had a wing span of 62 ft 2 in (18.95 m), was powered by two 600-hp (447-kW) Wright SR-1820-E Cyclone engines, and had open cockpits for a crew of three. Delivered to the US Army for testing at Wright Field on 20 March 1932, it was able to demonstrate a maximum speed of 197 mph (317 km/h) at 6,000 ft (1,830 m) when trials got under way in July of that year. For these trials the aircraft was allocated an experimental designation of XB-907.
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<p>Following discussions, the Martin 123 was returned to the factory in the autumn for modifications which included the installation of a front gun turret that, despite only carrying one machine gun, was the first to be installed in a US bomber. In addition to this work, more powerful 675-hp (503-kW) R-1820-19 engines were installed in long-chord NACA cowlings, and wing span was increased to 70 ft 7 in (21.51 m). When tested again by the US Army in late 1932, and then designated XB-907A, a speed of 207 mph (333 km/h) was recorded at 6,000 ft (1,830 m), a turn of speed which proved to be faster than that of any contemporary US fighters.
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<p>Not surprisingly, the US Army contracted for 48 production aircraft on 17 January 1933 and, at the same time, bought the XB-907A and redesignated it XB-10. Martin's production version (Martin 139) began to enter service in June 1934. It was a mid-wing monoplane of all-metal construction, except for fabric-covered control surfaces. The tailwheel-type landing gear was manually retractable, and the standard powerplant was two Wright Cyclones. The crew of four or five was accommodated in three separate cockpits with the bomb aimer/gunner in the nose, pilot and radio operator just forward of the wing leading edge, and gunners or a gunner and navigator in the aft cockpit. The nose position was covered by a cupola to permit deployment of the gun, and the other two cockpits each had a transparent canopy.
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<p>The first 14 production aircraft, with 675-hp (503-kW) R-1820-25 engines, were designated YB-10; the seven YB-12s which followed had 775-hp (578-kW) Pratt & Whitney R-1690-11 Hornet engines. The next 25 production B-12As also had the Hornet engines, and provision was made for an extra fuel tank to be carried in the bomb bay. Late procurement, in 1934 and 1935, resulted in 88 and 15 B-10Bs, respectively. Odd variants included a single B-10A with two turbocharged R-1820-31 Cyclones, one XB-14 with two 950-hp (708-kW) Pratt & Whitney YR-1830-9 Twin Wasps and, resulting from late modification programs, B-10Ms and B-12AMs equipped as target tugs. Several YB-10s and B-12As were modified by the US Army, provided with float installations for operation on coastal defense duties.
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<p>These Martin bombers remained in service with the USAAC until replaced by Boeing B-17s and Douglas B-18s in the late 1930s, but Martin also had some success with sales to foreign customers, building 189 for export. Of these, 120 went to the Netherlands East Indies, and these were among the first US-built bombers to see operational service at the beginning of World War II. Late examples from this export production had a continuous transparent canopy over the pilot's and rear gunner's cockpits, and were identified by Martin as Martin 166s.
<item><hi format=bold>Performance:</hi> maximum speed 213 mph (343 km/h) at 6,000 ft (1,830 m); cruising speed 193 mph (311 km/h); service ceiling 24,200 ft (7,375 m); range 1,240 miles (1,996 km)
<item><hi format=bold>Weights:</hi> empty 9,681 lb (4,391 kg); maximum take-off 16,400 lb (7,439 kg)
<item><hi format=bold>Dimensions:</hi> span 70 ft 6 in (21.49 m ); length 44 ft 9 in (13.64 m); height 15 ft 5 in (4.70 m); wing area 678 sq ft (62.99 m<su>2</su>)
<item><hi format=bold>Armament:</hi> three 0.30-in (7.62-mm) machine guns (one each in nose and rear turrets and one in ventral position), plus up to 2,260 lb (1,025 kg) of bombs carried internally
