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The Douglas O-35 was a twin-engined monoplane observation aircraft that was ordered in small numbers as a test aircraft, and that took part in the Air Corps air mail operation of 1934.
Douglas had produced a series of single engined biplane observation aircraft for the US Air Corps, and in 1929 developed the single engined monoplane O-31, receiving an order to produce two prototypes early in 1930. However 1929 also saw the appearance of a possible threat to the Douglas domination of observation aircraft in the shape of the Fokker XO-27, a twin engined monoplane.
Douglas approached the US War Department and asked for a contract to develop a twin engined observation aircraft of their own. The War Department agreed, and on 26 March 1930 issued a contract for the production of two prototypes - the XO-35 to be powered by geared Curtiss Conqueror engines and the XO-36 which was to use a direct drive engine.
Douglas produced an aircraft that was a mix of modern and outdated features. Its modern features included its all metal construction. Its engines were mounted in streamlined nacelles and the main undercarriage was retractable, folding back into the nacelles. The aircraft was designed to carry a radio, and the radio operator had an enclosed cabin.
In contrast the nose gunner, pilot and upper-mid gunner had open cockpits, with the pilot just ahead of the wing. The engine nacelle was carried well below the wing and was supported on a series of struts, which continued out to the mid point of the level section of the wing. The prototypes both used corrugated duralumin to cover the fuselage and tail surfaces.
By the time the XO-35 prototype was completed the XO-36 had been turned into a light bomber, the XB-7. The XO-35 was completed in the spring of 1931 and underwent a series of company tests at Santa Monica. It was delivered to the Air Corps at Wright Field on 24 October 1931, but by then an order had already been placed for five Y1O-35s and seven Y1B-7s.
The XO-35 prototype was damaged on 11 July 1932 but was repaired and returned to service. It took part in the air mail operations of 1934 (see below) and was eventually grounded on 28 October 1938 after flying for 999 hours.
The production aircraft were delivered between August and November 1932. All of them used geared engines, and the Y1O-35s were powered by either the 650hp V-1570-39 or the 675hp V-1570-33. They had smooth metal covering for the fuselage and were 11in longer than the prototype.
The O-35 was used by the 1st, 5th, 12th and 99th Observation Squadrons early in their service careers, in each case alongside other types.
In February 1934 President Roosevelt cancelled the private Air Mail contracts after an investigation into fraud. The Air Corps was unexpectedly given the task of carrying the mail, and only a couple of weeks to prepare. The five O-35s and the XO-35, along with the six remaining B-7s, were allocated to the Western Zone, and had the task of carrying mail in the area between the Pacific Coast and Cheyenne, Wyoming. The operation lasted from 20 February until 1 June. During that period four of the six B-7s were lost. All of the O-35s survived intact.
From 1935-37 the O-35 was used by the 88th Observation Squadron (later the 436th Bombardment Squadron). This squadron had all five O-35s in December 1935 when it took part in an air exercise in Florida designed to test the Air Corps' ability to cope with a hostile air force in the Caribbean.
The last O-35 was grounded in February 1939. The monoplane O-31 was actually a more successful design, leading to the O-43 and O-46. The last member of that family remained in service until 1946.
XO-35
Engine: Two Curtiss Conqueror V-1570-29 geared engines
Power: 600hp
Crew: 4
Span: 65ft
Length: 45ft
Height: 11ft 7in
Empty weight: 7,296lb
Loaded weight: 9,494lb
Maximum take-off weight: 10,254lb
Max speed: 178mph at sea level
Cruising speed: 156mph
Climb Rate: 8.4min to 10,000ft
Service ceiling: 21,750ft
Armament: Two flexibly mounted machine guns