The Martin Bombers
The bomber's role at that time was seen by the Army as that of coast defense. This had been formalized by the MacArthur-Pratt agreement on January 9, 1931, between the Army's Chief of Staff and the Chief of Naval Operations, which assigned Army air forces to defending the coasts of the continental United States and its overseas holdings, while Navy air forces would be free to move with the fleet on offensive missions. This arrangement continued the old tradition of concentrating Army efforts on coastal fortifications while the Navy would be the first offensive force. Seen in this perspective, the Army bombers were essentially long-range coastal guns.
The Martin Aircraft Company's Model 123 was developed in response to Air Corps pressure for a mid-wing all-metal monoplane. The stressed-skin monocoque fuselage had corrugated top and bottom surfaces with a deep belly and doors for the internal bomb bay. The wheels retracted into nacelles behind two 600-hp Wright SR-1820E Cyclones covered by anti-drag rings, and three open cockpits were provided, with room for a fourth crew member within the fuselage behind the bomb bay.
Known as the XB-907, the first version was flown February 26, 1932, at Baltimore, Maryland. Numerous difficulties delayed testing by the Army, but a July 1932 Wright Field report announced a 197-mph top speed and too-fast landing speeds. The aircraft was returned to the factory for rebuilding with a larger wing, flaps, and 675-hp R-1820F series Cyclones in full cowlings moved forward ahead of the wings.
The new XB-907A, later designated XB-10, first flew October 4, 1932. The first rotating transparent turret on a U.S. bomber protected the front gunner from the slipstream of a 207-mph speed. Not only were the Keystones now totally obsolete, but the Martin was faster than any pursuit plane then in Army service. The technical revolution accomplished in bomber design by all-metal monoplanes would now be forced on fighter design. Little wonder that the Collier Trophy was awarded in 1933 to Glenn L. Martin for this ship. Army engineers, however, complained that the prestigious award ignored the role the Air Corps played with its insistence on incorporating the newest ideas in the prototype.
The XB-10 prototype was purchased by the Army at the same time a contract approved January 24, 1933, ordered 48 production Martins at a unit cost of $50,840 each. (Costs had more than doubled in the decade since Martin's last bomber, the NBS-l.) These ships differed from the prototype in having a sliding canopy over the pilot's and rear gunner's cockpits and a simplified landing gear. Three .30-caliber Brownings with 500 rpg were located in the front turret, rear cockpit, and lower tunnel position. The bomb bay could accommodate two 1,130, three 624, or five 300-pound bombs, and accuracy was improved by provisions for the very secret Norden Mk XV bombsights that the Army began receiving via the Navy in April 1933. An external shackle could be fitted under the right wing for a 2,000-pound bomb.