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.