The Douglas Aircraft Corp.’s first observation monoplane was the XO-31, an all-metal, single-engine, straight-wing aircraft that won a contract from the U.S. Army Air Corps on 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, when President Franklin Roosevelt directed the Army Air Corps to take over from private contractors because of suspected improprieties in awarding contracts.