fuselage skins rather than the corrugated skins used on the prototype, while the fuselage was longer to adjust the aircraft's center of gravity. A revised fuel system was fitted, with more fuel carried in order to give the two-hour endurance specified by the Air Corps. More powerful Conqueror engines were fitted, with all aircraft using geared engines as used on the XO-35. Operational history The XO-35 was delivered to the Air Corps at its test center at Wright Field, Ohio on 24 October 1931, with the XB-7 following in July 1932. The twelve service test aircraft were completed between November 1932 and March 1933. The Y1B-7s equipped the 11th and 31st Bombardment Squadrons at March Field, California, while the Y1O-35s were issued to a number of Squadrons, including the 88th Observation Squadron at Brooks Field, Texas. One notable early use of the Y1B-7 was during a May 1933 exercise at Fort Knox, where it was able to outrun biplane fighters like the Boeing P-12, showing that the Air Corps needed more modern fighters. Despite the aircraft's superior performance to the elderly Keystone biplanes, the Douglas monoplane never entered mass production either in the observation or bomber roles because newer, more capable aircraft, such as the Martin B-10, were under development while a change in Air Corps policy meant that it was no longer interested in buying more twin-engined observation aircraft. In February 1934, US President Franklin D. Roosevelt cancelled all air mail contracts owing to a scandal relating to the award of contracts, thus initiating the Airmail Emergency, where the Air Corps was tasked with carrying the mail until new contracts could be awarded. The O-35s and B-7s were considered some of the least unsuitable aircraft in the Air Corps inventory for the air mail role, so the six remaining Y1B-7s and the five Y1O-35s, together with the XO-35, were assigned to air mail routes in the western United States, which included hazardous operations over the Rocky Mountains, the Cascade Range and the Sierra Nevada. By the time the Air Corps involvement in air mail flights ended in June 1934, the twelve O-35s and B-7s had flown 1,412 hours on mail flights, while four of the B-7s had been lost as a result of accidents. The last O-35s and B-7s were withdrawn from use in December 1938. Variants XO-35 Prototype twin-engined observation aircraft. Two geared Curtiss V-1570-29 engines driving three-bladed