02/28/2023. Remarks by Johan Visschedijk: "Powered by Curtiss V-1570-53 engines driving three-blade propellers, four YO-31As (serials 31-604 to 31-607) ordered under Contract AC-4326, were accepted by the USAAC between January and May 1932.

The corrugated metal skin fuselage and tail of the XO-31 and YO-31 was replaced by semi-monocoque construction with smooth metal covering and a canopy was fitted over the pilot's cockpit and the forward section of the observer's cockpit. The YO-31As also had revised tail surfaces, with the tail plane and elevators attached to the vertical fin and braced to the rear fuselage by supporting struts. A smoother contoured engine cowling was adopted, a spinner was fitted over the hub of the propeller, while the landing gear was fitted with wheel spats. The most important modification, however, was that affecting the span, area and geometry of the gull-wings as the parallel leading and trailing edges of the YO-31 wings gave way to the YO-31As wings of elliptical planform.

XYO-31A (31-604) at Wright Field (Johan Visschedijk Collection)

XYO-31A (31-604) in the Full Scale Tunnel at NACA Langley, note one man standing next to a pole beneath the tail of the aircraft, and another kneeling within the wind tunnel (Johan Visschedijk Collection)

The first YO-31A (serial 31-604) underwent tests, temporarily redesignated XYO-31A, at Wright Field with the USAAC, then at Langley with NACA and finally back at Santa Monica with Douglas. It was used to test four different wing positions and led to a progressive growth of the vertical tail surfaces culminating in a unit with a tall pointed fin and inset rudder, the temporary fin extension was riveted to the original vertical surfaces.

Another YO-31A was tested in April 1932 against the Curtiss YO-40A but the YO-31A was found inferior to the Curtiss type in almost all respects; however, the experimental Wright Y1R-1820-F powering the YO-40A was extremely unreliable and, on May 20, 1932, the Curtiss crashed and left the field to Douglas.

Following completion of Service tests, the four YO-31As were redesignated O-31As.

Five Y1O-31A aircraft (serials 32-291 to 32-295) were ordered under Contract AC-4534 on August 26, 1931. Before completion these aircraft were redesignated Y1O-31Cs but, eventually, they were completed as Y1O-43s) in 1933."