The Douglas Y1B-7, an American bomber aircraft from the 1930s, was the first monoplane in the US to be designated as a B-bomber.
The Army Air Corps of the United States chose to try out monoplanes because they were more practical and less expensive than biplanes.
During the time that the XB-7 was ordered, Douglas Aircraft was testing it as an observational plane.
In order to maintain its role as the primary supplier of observation aircraft to the United States Army Air Corps, the Douglas Aircraft Company designed a new twin-engine monoplane observation aircraft in 1929-1930 to compete with the advanced Fokker XO-27.
The Air Corps ordered two prototypes, the XO-35 and XO-36, on March 26, 1930, which were identical except for their engines.
The XO-35 had geared Curtiss V-1570-29 Conqueror engines while the XO-36 used direct-drive V-1570-23, both producing 600 hp (450 kW) each.
The Douglas design featured high-mounted gull wings with engines suspended in streamlined nacelles under the wings by bracing struts.
A retractable tailwheel undercarriage was fitted, with main wheels retracting into the engine nacelles.
The semi-monocoque fuselage was covered in corrugated duralumin and housed a crew of four, including a nose gunner/observer, a pilot, an upper gunner, and a radio operator.
The armament consisted of two .30 in machine guns.
Both the Fokker and Douglas prototypes had much greater projected performance than the Keystone biplanes that equipped the Air Corps light bomber squadrons.
Therefore, both companies were instructed to complete one of their prototypes as a bomber, with the XO-36 being redesignated the XB-7 and having provision to carry up to 1,200 lb (540 kg) of bombs on racks under the fuselage.
The XO-36 made its first flight at the Douglas factory in Santa Monica, California in the spring of 1931.
The Air Corps then placed an order for a small batch of service test aircraft on August 22, 1931, consisting of seven Y1B-7 bombers and five Y1O-35 observation aircraft.
These differed from the prototypes by having smooth fuselage skins and a longer fuselage to adjust the aircraft’s centre 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.
All aircraft used more powerful Conqueror engines, with geared engines as used on the XO-35. Variants XO-35 Prototype twin-engine observation aircraft. Two 600 hp (450 kW) geared Curtiss V-1570-29 engines driving three-bladed propellers. One built. XO-36 Prototype twin-engine observation aircraft. Two 600 hp (450 kW) direct-drive Curtiss V-1570-23 engines driving two-bladed propellers. Redesignated XB-7. One built. XB-7 Prototype light bomber, with two direct-drive 600 hp V-1570-23 engines driving two-bladed propellers. One built. Y1O-35 Service test batch of observation aircraft, powered by 650 hp (480 kW) V-1570-39 or 675 hp (503 kW) V-1570-53 engines. Five built. Y1B-7 Service test batch of bombers, powered by 640 hp (480 kW) V-1570-33 or 675 hp V-1570-53 engines. Seven built. Specifications Length 45 ft 11 in (14.00 m) Wingspan 65 ft (20 m) Height 11 ft 7 in (3.53 m) Wing area 621.2 sq ft (57.71 m2) Empty weight 5,519 lb (2,503 kg) Gross weight 9,953 lb (4,515 kg) Max take-off weight 11,177 lb (5,070 kg) Powerplant 2 × Curtiss V-1570-53 Conqueror V-12 liquid-cooled piston engines, 675 hp (503 kW) each Propellers 3-bladed metal propellers Performance Maximum speed 182 mph (293 km/h, 158 kn) Cruise speed 158 mph (254 km/h, 137 kn) Range 411 mi (661 km, 357 nmi) Ferry range 632 mi (1,017 km, 549 nmi) Service ceiling 20,400 ft (6,200 m) Time to altitude 10,000 ft (3,000 m) in 8 minutes 42 seconds Wing loading 16 lb/sq ft (78 kg/m2) Power/mass 0.135 hp/lb (0.222 kW/kg) Armament Guns 2 × 0.30 in (7.6 mm) machine guns Bombs 1,200 lb (540 kg) of bombs carried beneath the fuselage.
Sources McDonnell Douglas aircraft since 1920, Volume 1-René J Francillon. San Diego Air and Space Museum Archive. The National Museum of the U.S. Air Force.