YB-10, YB-12, XB-14, BT-32
The first Martin off the production line, a YB-10, was delivered to Wright Field in November 1933 with 675-hp R-1820-17 Cyclones; 14 YB-10s had been delivered by April 1934. They can be distinguished from their successors by an air intake atop the engine cowling. A single YB-l0A delivered in June 1934 had experimental turbo-supercharged R-1820-31 Cyclones, and top speed increased from 202 mph at 10,000 feet to 236 mph at 25,000 feet. This Cyclone-supercharger combination, after much development, would become standard for all the B-17s built after 1938.
Pratt & Whitney Hornets of 700 hp were used on seven YB-12 and 25 YB-12A models, the first appearing in February 1934, with the air intakes on the nacelle's port side. These intakes were on top, behind the cowl, on the B-12As,
first seen in June 1934 and introducing provisions for increasing fuel tankage to 580 gallons and sealed flotation compartments in the wings. A plan for a dozen YB-13s with R-1860-17 Hornet Bs was canceled, but one XB-14 in May 1934 introduced the Twin Wasp, YR-1830-1, which would become the power plant for World War II's B-24 bombers.
Adapting the Martin to a multi-seat fighter, of the style then favored in France, was discussed in March 1933. Gun turrets fore and aft, along with a retractable belly turret, were considered. Later, XA-15 attack and XO-45 observation versions of the YB-10 were proposed, but these were also dropped from plans. One B-12A was fitted with twin floats to set a seaplane speed record on August 24, 1935.
Most of the Martins were based at March Field, California. Ten YB-l0s demonstrated their serviceability with a survey flight to Alaska in July 1934, and then the YB-10s remained at March for the 19th Bomb Group. In December 1934 the Hornet-powered B-12/12A versions went with the 7th Bomb Group to the new Hamilton Field near San Francisco.
The main production version was the B-l0B, with 750-hp R-1820-23 Cyclones, intakes atop the nacelle, and behind them the exhaust pipes that were moved from the lower nacelle. A June 28, 1934, contract for 81 B-l0Bs was later increased to 103, with two more assembled by the Army from spare parts. The first B-l0B arrived at Wright Field in July 1935, where it remained for type tests, and quantity deliveries to Langley Field began in December 1935, at a cost of $55,299 each. The last B-10B from Martin was completed on August 8, 1936.
Martins reached all Army bomber groups, which in 1936 included the 2nd Bomb Group at Langley and the 9th at Mitchel Field with B-l0Bs on the East coast, and the 7th (B-l0B and B-12) and l9th (YB-10 and B-l0B) in California. Most of the B-12As went to Hawaii's 5th Composite Group in 1936. When Douglas B-18s began arriving in 1937, B-l0Bs became available to fill out the 6th Composite Group in the Canal Zone, and the last squadron to get B-l0Bs was the 28th in the Philippines, where they were used until 1940. By that time most stateside Martins were relegated to target-towing and utility work, and designated B-10M.
With such a performance record, winning export orders would be easy, but the Army forbade release of the Martin bomber until its own deliveries were complete. In the meantime, attempts at export sales were made by less advanced designs.
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