The Douglas O-31 was the first in a series of monoplane observation aircraft produced to replace the existing Douglas biplane aircraft, and eventually led to production orders for the O-43 and O-46.
During the 1920s and early 1930s Douglas produced a series of successful observation biplanes, starting with the Douglas O-2 of 1924. By the end of the 1920s it was clear to Douglas that the biplane versions would soon be outclassed, and the company decided to take advantage of its experience with monoplane aircraft to produce a new observation type. The company produced a design for an all-metal aircraft with high mounted gull wings. This interested the US War Department, and on 7 January 1930 Douglas was awarded Contract AC-2822, to produce two experimental aircraft.
The first aircraft made its maiden flight in December 1930, with the designation XO-31. It was a gull-wing monoplane, with the wing wire braced above to a four strut cabane mounted above the fuselage, and below to the fuselage. It had good clean lines, but used corrugated duralumin as the skin behind the engine cowling. It was powered by a 600hp Curtiss Conqueror V-1570-25 V-12 liquid cooled engine and a split axle fixed undercarriage. The crew of two sat in open tandem cockpits. It was originally built with a fixed incidence tail plane.
A number of problems appeared in the manufacturer's trials. The radiator installation caused parasitic drag, handling was poor and there was directional instability. A series of efforts were made to solve these problems. The first changes saw the radiator moved back and a variable incidence tail plane installed. In this configuration the aircraft went to the USAAC for service trails in February 1931. The drag and handling problems had been largely eliminated, but the directional instability remained. A series of alternative tails were tested out on the XO-31 between 1931 and 1937, including the use of auxiliary fins on the horizontal tail plane and a number of larger tails.
The second prototype was delivered to the Air Corps in May 1931 as the YO-31. It was powered by a geared Curtiss V-1570-7 engine which required a 3in longer engine cowling, and was built with a modified vertical fin.
On 23 June 1931 Douglas was given a contract to produce another six O-31s (Contract AC-4326). Four of these aircraft were delivered between January and May 1932 as the YO-31A. This saw a number of major changes in the design. The wings were given elliptical edges. The fuselage was of semi-monocoque construction with a small metal covering. A canopy was fitted over the pilot's position and the front of the observer's cockpit. The engine was given a new cowling and the propeller had a streamlined spinner. The horizontal tail surfaces were raised from the fuselage to the base of the vertical tail, and supported with struts. These aircraft were used for extensive service tests and also for development work. The first of the four was used to test four different wing positions and a series of increasingly tall tails. The final version had a tall pointed fin with an inset rudder. After the end of the trials these aircraft entered regular service as the O-31A.
The fifth aircraft from the order was completed as the YO-31C. This was given a ventral bulge under the rear cockpit to give the observer more standing room when using his flexibly mounted machine gun. It was also give a cantilevered undercarriage in place of the earlier split axle type. After its trials were over the YO-31C became the O-31C.
The sixth aircraft from the order was completed as the YO-31B. This was an unarmed two-seat staff transport, built for use by the Militia Bureau of the War Department. It used a 600hp Curtiss V-1570-29 engine and had a sliding canopy. This aircraft was later given the larger tail and modified wings tested on the YO-31A. It served as the O-31B and remained in use until May 1942.
On 26 August 1931 another five aircraft were ordered (Contract AC-4534). At first these aircraft were given the designation Y1O-31C, but they were completed as the Y1O-43.
Three of the O-31As and the O-31Cs served together with a series of Observation Squadrons, starting with the 1st, then moving on to the 12th, 22nd and 99th. During 1940 the last two surviving aircraft were used as instructional airframes.
Engine: Curtiss V-1570-7
Span: 46ft 4in
Length: 33ft 5in
Height: 10ft 7in
Empty weight: 3,496lb
Loaded weight: 4,654lb
Maximum take-off weight:
Max speed: 182.5mph
Climb Rate: 1,700ft/ min
Service ceiling: 24,300ft