The Douglas B-7 was the company’s first monoplane bomber, and although it wasn't produced in large numbers did help the US Army Air Corps convert from the older biplanes.
The B-7 was originally designed as an observation aircraft, the XO-36. This was one of a pair of observation aircraft, the XO-35 and XO-36, that were developed after the US War Department placed an order 1929 for two prototypes of the twin-engined Fokker XO-27 monoplane. Douglas had produced a long series of single-engined biplane observation aircraft, and early in 1930 had been awarded a contract for their first observation monoplane, the XO-31, but the arrival of the twin-engined Fokker aircraft worried them.
Douglas responded by asking the War Department to fund the development of their own twin-engined monoplane observation aircraft. The War Department agreed to the proposal, and on 26 March 1930 issued Douglas with a contract to design and produce two prototypes. The two aircraft were to be identical apart from the engines. The XO-35 was to use the Curtiss GIV-1570C Conqueror, a geared engine that would drive a three blade propeller. The XO-36 was to use the direct drive V-1570-23 (V-1570C in civil service), and two blade propellers.
The XO-35 was completed as an observation aircraft, but the XO-36 became the basis for the Y1B-7. During the development process it became clear that the new Douglas design would outperform the Keystone bombers then in use with the Air Corps (the B-3A, B-4A, B-5A and B-6A, all twin engined biplanes). The Air Corps decided to convert the incomplete XO-36 into a light bomber, as the XB-7. Fokker was ordered to carry out a similar conversion of the second XO-27, which became the Fokker XB-8.
The XO-35 was ready in the spring of 1931, but underwent lengthy tests at Santa Monica before being delivered to the Air Corps at Wright Field on 24 October 1931. The XB-7 took longer to test, and wasn't delivered until July 1932. The early tests must have been satisfactory, for the Air Corps ordered seven Y1B-7s and five Y1O-35s on 22 August 1931. All of these aircraft were delivered between August and November 1932.
The XB-7 was very much an interim design, falling between the established biplanes and more mature monoplanes. It inherited the gull wing configuration developed on the XO-31, but while the XO-31 resembled later parasol winged observation aircraft the XB-7 looked very much like a biplane with the lower wing removed (less so in the air with the undercarriage retracted). The gull wing was raised some way above the top of the fuselage. The engines were carried in nacelles that were level with the fuselage, and that were connected to the rest of the aircraft by a series of struts (including level struts connecting the base of the nacelle to the base of the fuselage and the main wing struts, which linked the outer base of the nacelle to the mid-point of the level section of the wing. Smaller struts went up to the wing. The retractable undercarriage was carried in the lower part of the nacelles.
The aircraft was of all metal construction. The prototypes of both the XB-7 and XO-35 were covered with corrugated duralumin, although this was replaced with a smooth covering on production aircraft. The mix of modern and outdated features continued with the crew accommodation. The two gunners and the pilot had open cockpits, with one gunner in the nose and one amidships, while the pilot was just ahead of the wing. In contrast the radio operator had an enclosed cockpit behind the pilot.
The seven Y1B-7s were based at March Field, California, and were operated by the 11th and 31st Bombardment Squadrons, in both cases alongside a variety of other aircraft.
The official USAAF history of combat squadrons also records the 12th Observation Squadron (Brooks Field, Texas) and 88th Observation Squadron (Hamilton Field, California, later the 436th Bombardment Squadron) as having used the B-7 (alongside other types). These squadrons also used the O-35, so probably operated tiny numbers of B-7s as observation aircraft. The 12th Observation Squadron had at least one Y1B-7 during the 1934 Air Mail operations.
Although the B-7 was a great improvement on the biplanes it replaced, by the time it entered service it was already somewhat outdated. 1932 also saw the entry into service of the Boeing B-9, an all-metal low winged monoplane that was slightly faster than the B-7, had a longer range and could carry nearly twice the payload. The Martin B-10 was further ahead.
Some of the B-7s took part in an air defence exercise in May 1933, forming the bomber force used to test out the Air Corps' early warning, anti-aircraft and interceptor systems. The early warning system worked well, but on at least one occasion the Air Corps P-12s were unable to keep up with the B-7 they had intercepted.
In February 1934 the Roosevelt administration cancelled the existing commercial air mail contracts after investigating collusion and fraud. The Air Corps was given the job of carrying the mail and had to create a massive cargo carrying network from scratch. Smaller aircraft were used for short local routes, while the bombers flew the longer routes with heavier loads. The B-7 was faster than the Keystone biplanes, but not as good at the task as the Martin YB-10.
The Air Corps allocated the XO-35, the six surviving B-7s and the five O-35s to the Western Zone, between Cheyenne, Wyoming and the Pacific Coast. This would be a very costly operation for the B-7s. One was lost on 16 February 1934, on a familiarization flight before the air mail operation had even begun. Three more B-7s were lost (but no O-35s), so at the end of the operation only two of the production B-7s survived. The surviving aircraft were soon moved to secondary roles and all were grounded by the summer of 1939.
Engine: Two Curtiss Conqueror V-1570-53
Power: 675hp each
Height: 11ft 7in
Empty weight: 5,519lb
Loaded weight: 9,953lb
Maximum take-off weight: 11,177lb
Max speed: 182mph at sea level
Cruising speed: 158mph
Climb Rate: 8.7mins to 10,000ft
Service ceiling: 20,400ft
Range: 411 miles normal, 632 miles maximum
Armament: Two flexibly mounted 0.3 machine guns
Bomb load: 1,200lb bombs under fuselage