In early 1930, the Douglas aircraft company submitted a proposal to the Army for a twin-engined observation plane. It was designed to compete with the Fokker XO-27, two examples of which had been ordered in June of 1929.
The Douglas proposal was for a monoplane with high-mounted braced gull wings and metal construction with corrugated duralumin covering on the fuselage and tail surfaces. It was to be powered by a pair of Curtiss Conqueror twelve-cylinder liquid-cooled Vee engines that were housed in nacelles attached underneath the wing by a series of struts. The main undercarriage members retracted backwards into the engine nacelles, but the lower portion of the wheels remained exposed in order to reduce the amount of damage in the event of a wheels-up landing. Four crew members were to be carried--an observer/gunner in an open cockpit in the nose firing a single 0.30-inch machine gun, a pilot in an open cockpit just ahead of the wing, a gunner in an open dorsal cockpit in the rear fuselage firing a single 0.30-inch machine gun, and a radio operator in an enclosed cabin admidships.
On March 26, 1930, the Army ordered two example of the Douglas proposal. One was designated XO-35 (30-227) and the other XO-36 (30-228). The two planes were to be almost identical to each other, with the primary difference being that the XO-35 had two geared 600 hp Curtiss GIV-1570C (military designation V-1570-29) Conquerors driving three-bladed propellers and the XO-36 had two direct-drive 600 hp Curtiss V-1570C (military designation V-1570-23) Conquerors driving two-bladed propellers. Since the propellers of the XO-36 were of smaller diameter than those of the XO-35, the engine nacelles of the XO-36 were to be mounted 8 inches closer to the aircraft centerline.
The performance of the XO-35/36 promised to greatly exceed that of the lumbering Keystone biplanes that were at that time the standard USAAC light bombers. Consequently, the Army decided to have the XO-36 completed as a light bomber rather than as an observation plane. It was assigned the designation XB-7, and was to have been equipped with racks for 1200 pounds of bombs underneath the fuselage. At the same time, the Army ordered that the second Fokker XO-27 be completed as a light bomber under the designation XB-8.
The XO-35 flew for the first time in the spring of 1931. It was delivered to Wright Field on October 24, 1931. The XB-7 was delivered to Wright Field in July of 1932. The XB-7 had racks for 1200 pounds of bombs underneath the fuselage. Both planes had corrugated metal fuselage and tail coverings. The tailplane was supported by wire bracing. It carried a crew of four--pilot, copilot, nose gunner, and rear gunner.
On August 22, 1931, the USAAC ordered seven Y1B-7 bombers and five Y1O-35s. Serials were 32-308/314 and 32-315/319 respectively. These were delivered between August and November of 1932. Both the bomber and observation models standardized on geared versions of the Conqueror engine. The Y1B-7 was powered by a pair of 640 hp V-1570-33 or 675 hp V-1570-52 engines, whereas the Y1O-35 was powered by a pair of 650 hp V-1570-39 or 675 hp V-1570-53 engines.
The service-test Y1B-7 aircraft differed from the prototypes in having smooth rather than corrugated metal covering on their fuselages and in having fabric covering for their movable tail surfaces. The length was increased from 45 feet to 45 feet 11 inches, and an adjustable tab was added to the rudder. The tailplane was now supported by metal struts rather than by wires. Fuel capacity was increased by 116 US gallons. The fuel distribution system was modified and the engine controls and the oil cooler were improved.
Shortly afterward, the Army lost interest in twin-engined observation aircraft and no production examples of the Y1O-35 were ordered. In addition, the Y1B-7 was rapidly made obsolete by advances in bomber technology (such as the Martin B-10), and no production was ordered for this version either.
The seven Y1B-7s were assigned to the 11th and 31st Bombardment Squadrons at March Field in California, becoming the Army's first monoplane bomber to enter service. They were later redesignated B-7. One was lost in a crash during its first year of operation. The five Y1O-35s (later redesignated O-35) entered service with Observation Squadrons at Crissy and Mitchel Fields.
On February 9, 1934, President Franklin Roosevelt cancelled all air mail contracts with civilian carriers and ordered the Army Air Corps to take over the flying of the air mail. The Army had few aircraft that ware suitable for this mission, but a motley collection of bombers, transports, observation planes, and even fighters were assembled. Among the planes assigned to air mail duty were the XO-35, the five O-35s, and the six surviving B-7s. These planes were assigned the mission of carrying the air mail from Cheyenne, Wyoming to the Pacific coast, a particular dangerous route since it involved flying over a lot of mountains. By June 1, 1934, when the Army Air Corps finally stopped flying the air mail, four B-7s had been lost in accidents. However, the XO-35 prototype and all five of the O-35s survived the air mail duty.
Following the completion of the air mail duty, the O-35/B-7s which had survived returned to more conventional military roles, and they remained flying until nearly the end of the 1930s, despite their obsolescence. The XO-35 was surveyed on October 28, 1938, with the XB-7 being surveyed six months later. The last of the O-35s was surveyed in February of 1939. The two B-7s which had survived the air mail ordeal were surveyed in 1938/39. No O-35/B-7 bombers survive today.
Douglas XO-35 30-227 Douglas XO-36 --> XB-7 30-228 Douglas Y1B-7 32-308/314 Douglas Y1O-35 32-315/319
Specification of the Douglas Y1B-7
Two 640 hp Curtiss V-1570-33 or 675 hp V-1570-52 Conqueror twelve-cylinder liquid-cooled Vee engines. Maximum speed: 182 mph at sea level, 177 mph at 5000 feet. Cruising speed: 155 mph. Landing speed 78 mph. Climb to 5000 feet in 3.7 minutes. Climb to 10,000 feet in 8.7 minutes. Service ceiling 20,400 feet, absolute ceiling 21,800 feet. Normal range 411 miles, maximum range 632 miles. Weights: 5519 pounds empty, 9953 pounds loaded, 11,177 pounds maximum Dimensions: wingspan 65 feet, length 45 feet 11 inches, height 11 feet 7 inches, wing area 621.2 square feet. Armed with two 0.30-inch machine guns, one in a flexible nose position and the other in a flexible dorsal position. 1200 pounds of bombs could be carried on racks underneath the fuselage.