Douglas' first observation monoplane was the XO-31, an all-metal, single-engine, straight-wing aircraft that won a contract Jan. 7, 1930.
The straight-wing, single-engine O-43 followed the XO-31. It had a deeper fuselage and taller vertical surfaces with an inset rudder. Twenty-four O-43s were built.
Douglas then produced 90 further-improved, single-engine, straight-wing O-46s, built for the U.S. Army Air Corps. The enclosed cockpit O-46 series was the last observation aircraft built by Douglas and was the company's most successful Air Corps program since the O-38 biplane series.
During the same period, Douglas designed the gull-winged, twin-engine XO-35. Similar to its XB-7 bomber counterpart, the XO-35 had two Curtiss Conqueror liquid-cooled engines enclosed in nacelles attached under the wing. The O-35 was never produced in quantity, but six Y1B-7s, serving with the Army Air Corps, delivered airmail to the western zone during the 1934 airmail emergency.
||45 feet 9 inches
||34 feet 6 inches
||10 feet 8 inches
||One 725-horsepower Pratt & Whitney R1535 air-cooled radial engine