The Douglas O-46 was the main production version of the Douglas family of monoplane observation aircraft, and the first to use a radial engine.
The family began with the gull-winged O-31, which was produced in small numbers in the early 1930s, and was mainly used for further research. This was followed by the O-43, the first production version. This used a parasol wing, wire braced to a four-strut cabane above and the fuselage below the wings. It had a semi-monocoque fuselage, and was produced in two different versions with different tail installations.
In 1934 an order was placed for twenty four O-43As, but the last of these aircraft was completed as the XO-46, which was delivered in October 1934. Two major changes were introduced on this version. First was the use of a 725hp Pratt & Whitney R-1535-7 fourteen cylinder air cooled radial engine in place of the inline engine used on the O-31 and O-43. The second was the introduction of parallel streamlined struts below the wings, replacing the wire bracing and cabane of the earlier versions.
The XO-46 was judged to be a success and it was followed by an order for 71 O-46A aircraft, placed on 29 April 1935 (Contract AC-7342). This was modified twice, with seventeen aircraft added on 31 October 1935 and two on 6 March 1936, bringing the total to 90. The first 88 aircraft were ordered for the USAAC and the final two for the National Guard. All ninety were delivered between May 1936 and April 1937.
The O-46A was similar to the prototype XO-46, but with the engine moved forward by 8.5in to shift the centre of gravity and the cockpit canopy faired into the vertical tail fin.
One further variant was proposed - the YO-48 with a 775 Wright R-1670-3 radial engine. A contract for one prototype was issued but this was cancelled before the aircraft could be built.
The O-46A entered service in 1936, and was used by a number of Observation Squadrons. In 1938 the corps area commanders were told to conduct service tests of the O-46 and O-47 in field conditions to see if the increases in speed, weight and complexity of the observation aircraft had made them less suited to their role. At this stage the O-46 performed OK, and the commander of the 97th Observation Squadron reported that his aircraft had performed well. However a 1939 report from Fort Lewis described the O-46 as a 'very nice fair weather cross country aircraft for a pilot who is in no hurry', too slow to outrun enemy fighters, too heavy to out-manoeuvre them, too heavy for wet fields, too slow to take off for use for enclosed fields and too large to hide under tress.
Most went to the National Guard in the late 1930s, but the 2nd Observation Squadron, based at Nichols Field, Luzon, still had some when the Japanese attacked in December 1941. As a result these O-46As became the only members of the Douglas monoplane observation series to see combat.
Some of the surviving O-46As were transferred to training units in 1942. Amongst them were the 26th and 70th Reconnaissance Groups, which also flew some anti-submarine patrols after the US entry into the war (although probably not with their O-46s). One survived the war and entered the collection of the USAF Museum at Wright-Patterson AFB.
Engine: Pratt & Whitney R-1535-7 radial engine
Span: 45ft 9in
Length: 34ft 6 3/4in
Height: 10ft 8in
Empty weight: 4,776lb
Maximum take-off weight: 6,639lb
Max speed: 200mph at 4,000ft
Climb Rate: 1,765ft/ min
Service ceiling: 24,150ft
Range: 435 miles
Armament: One fixed forward firing 0.3in machine gun in leading edge of starboard wing; one 0.3in gun on observer's cockpit