The Douglas O-31 was the initial monoplane observation straight-wing aircraft developed by the Douglas Aircraft Company and utilized by the United States Army Air Corps.
To maintain its position as the primary supplier of observation aircraft to the USAAC, Douglas proposed a high-wing monoplane successor to the O-2.
A contract was executed on January 7, 1930, for two XO-31 prototype aircraft, with the first being flown in December of the same year.
The XO-31 was a fabric-covered gull-wing monoplane with a slim corrugated dural-wrapped fuselage, similar to the Thomas-Morse O-19, and featured a tandem arrangement of open cockpits for the pilot and observer.
It was equipped with one 675 hp (503 kW) Curtiss GIV-1570-FM Conqueror V-12 engine and fixed landing gear with provision for large wheel fairings.
The XO-31 experienced directional instability, and various fins, auxiliary fins, and rudder shapes were experimented with to address the issue.
The second aircraft was completed as the YO-31, with a geared Curtiss V-1570-7 Conqueror engine and an enlarged fin, a 3″ longer cowling, and a two-blade, dextrorotatory propeller.
Four YO-31A aircraft were delivered during early 1932 and underwent significant modifications, including an elliptical wing planform, a new tail assembly, a smooth semi-monocoque fuselage, a three-blade propeller, and a canopy over the cockpits.
The aircraft appeared with a variety of tail units, with the final version (five built) designated O-31A featuring a very pointed fin with an inset rudder.
The single YO-31B was an unarmed staff transport, and the sole YO-31C, converted from YO-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.
In 1931, five Y1O-31C service-test aircraft were ordered 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. Variants XO-31 two built, Curtiss V-1570-25 Conqueror engine YO-31 revised XO-31, length increased to 33 ft 5 in (10.19 m), Curtiss V-1570-7 engine. YO-31A 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 engine. YO-31B one built, re-designated O-31B, Curtiss V-1570-29 engine YO-31C YO-31A with cantilever gear, Curtiss V-1570-53 engine Y1O-31C five built, wingspan increased to 45 ft 11 in (14 m), became the Y1O-43, Curtiss V-1570-53 engine. Specifications YO-31C Crew 2 Length 34 ft 5 in (10.49 m) Wingspan 45 ft 11 in (14.00 m) Height 11 ft 6.75 in (3.5243 m) Wing area 339.9 sq ft (31.58 m2) Empty weight 3,888 lb (1,764 kg) Gross weight 4,982 lb (2,260 kg) Powerplant 1 × Curtiss GIV-1570-FM Conqueror (V-1570-53) Performance Maximum speed 195 mph (314 km/h, 169 kn) at sea level Cruise speed 160 mph (260 km/h, 140 kn) Range 260 mi (420 km, 230 nmi) Service ceiling 22,700 ft (6,900 m) Rate of climb 1,515 ft/min (7.70 m/s) Time to altitude 10,000 ft (3,000 m)7 minutes 12 seconds Wing loading 14.6 lb/sq ft (71 kg/m2) Power/mass 0.12 hp/lb (0.20 kW/kg) Armament Guns 1 fixed and 1 flexible 0.30 Cal (7.62 mm) Browning machine gun.
Sources Air Mobility Command Museum. McDonnell Douglas aircraft since 1920: Volume I-René J Francillon. San Diego Air and Space Museum Archive. The National Museum of the U.S. Air Force.
