|Manufacturer||Douglas Aircraft Company|
|Primary user||United States Army Air Corps|
|Developed into||Douglas O-43|
Anxious to retain its position as chief supplier of observation aircraft to the USAAC, Douglas developed a proposal for a high-wing monoplane successor to the O-2. A contract was signed on January 7, 1930 for two XO-31 prototype aircraft, the first of them being flown in December of the same year. A fabric-covered gull-wing monoplane, the XO-31 had a slim corrugated dural-wrapped fuselage, similar to the Thomas-Morse O-19, carrying a tandem arrangement of open cockpits for the pilot and observer. It had one 675 hp (503 kW) Curtiss GIV-1570-FM Conqueror Vee engine and fixed landing gear with provision for large wheel fairings.
The XO-31 suffered from directional instability and experiments were made with various fin, auxiliary fins, and rudder shapes, in an effort to cure the problem. The second aircraft was completed as the YO-31, with a geared Curtiss V-1570-7 Conqueror engine and an enlarged fin, 3" longer cowling, and a two-blade, dextrorotatory propeller. Four YO-31A aircraft delivered during early 1932 were modified radically with an elliptical wing planform, a new tail assembly, a smooth semimonocoque fuselage, three-blade propeller, and a canopy over the cockpits. The aircraft appeared with a variety of tail units, the final version (five built) designated O-31A featured a very pointed fin with an inset rudder. The single YO-31B was an unarmed staff transport and the sole YO-31C converted from Y1O-31A had cantilever main landing gear, and a ventral bulge in the fuselage, which enabled the observer to operate his single 0.3-in (7.62 mm) machine-gun more effectively from a standing position.
Five Y1O-31A service-test aircraft were ordered in 1931, and delivered to the USAAC in early 1933 designated Y1O-43. They differed from the final configuration of the O-31A, with a wire-braced parasol wing, and a new fin and rudder. (See Douglas O-43)
Data from: "U.S. Army Aircraft 1908-1946" by James C. Fahey, 1946, 64pp.
- two built, Curtiss V-1570-25 Conqueror engine
- revised XO-31, length increased to 33 ft 5 in (10.19 m), Curtiss V-1570-7 Conqueror engine
- five built, re-designated O-31A, fuselage construction changed to a built-up semi-monocoque structure of flat sheets, length increased to 33 ft 11 in (10.34 m), Curtiss V-1570-53 Conqueror engine
- one built, re-designated O-31B, Curtiss V-1570-29 Conqueror engine
- YO-31A with cantilever gear, Curtiss V-1570-53 Conqueror engine
- five built, wingspan increased to 45 ft 11 in (14 m), became the Y1O-43, Curtiss V-1570-53 Conqueror engine
Data from "United States Military Aircraft Since 1909" by F. G. Swanborough & Peter M. Bowers (Putnam New York, ISBN 0-85177-816-X) 1964, 596 pp.
- Crew: 2
- Length: 33 ft 10 in (10.32 m)
- Wingspan: 45 ft 8 in (13.92 m)
- Height: 11 ft 9 in (3.58 m)
- Wing area: 340 ft2 (31.6 m2)
- Airfoil: curved planform
- Empty weight: 3,751 lb (1,701.5 kg)
- Loaded weight: 4,635 lb (2,102.4 kg)
- Powerplant: 1 × Curtiss GIV-1570-FM (Curtiss V-1570-53 Conqueror) Vee in-line, 600 hp (447 kW)
- Maximum speed: 190 mph at sea level (305.8 km/h)
- Cruise speed: 168 mph (270.4 km/h)
- Service ceiling: 22,700 ft (6919 m)
- Rate of climb: 1,515 ft/min (461.8 m/min)
- 1 fixed and 1 flexible 0.30 cal (7.62 mm) Browning machine gun
- "U.S. Army Aircraft 1908-1946" by James C. Fahey, 1946, 64pp.
- "United States Military Aircraft Since 1909" by F. G. Swanborough & Peter M. Bowers (Putnam New York, ISBN 0-85177-816-X) 1964, 596 pp.
- "The Complete Encyclopedia of World Aircraft" cover Editors: Paul Eden & Soph Moeng, (Amber Books Ltd. Bradley's Close, 74-77 White Lion Street, London, NI 9PF, 2002, ISBN 0-7607-3432-1), 1152 pp.
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