The Douglas O-25 was developed from the O-2H observation biplane, and had the Liberty engine of the original aircraft replaced by a Curtiss Conqueror. The O-25 was a major production version of the aircraft and a total of eighty four were built (including the prototype).
The O-2 had first emerged as a Liberty powered observation biplane with equal span un-staggered wings. The O-2H was a significant development, with a modified fuselage and tail and staggered unequal span wings. It retained the Liberty engine used in the O-2.
The O-25 was based on the O-2H, but used a Curtiss Conqueror V-1570 inline engine in place of the Liberty engine. Engine power thus increased from around 400hp to 600hp. The O-2H had used a vertical radiator mounted above the engine, but the O-25 reverted to the configuration of the O-2, with a tunnel radiator under the Conqueror engine.
Like the O-2 the O-25 was of mixed construction, with a welded steel tube structure for the fuselage, wooden wings and fabric covered apart from on the forward fuselage, which used aluminium panels. It carried two 0.30in machine guns - one fixed forward firing and one on a flexible mount in the rear cockpit. The O-25 saw the introduction of a new mounting for the rear gun. The O-25 had rigid inter-wing struts and similar split axle undercarriage to the O-2H.
The prototype O-25 was built using an airframe late in the production run of the O-2H (29-164). It was powered by a 600hp Curtiss Conqueror V-1570-5 liquid cooled engine. The O-25 was later given a Prestone cooled V-1570-27 engine and redesignated as the XO-25A.
The O-25A was heavier than the earlier O-2 or O-2H, but its more powerful engine meant that its performance was significantly better. Top speed rose from 134.5mph on the O-2H to 156.6mph, while service ceiling rose by over 5,000ft to 22,180ft.
The O-25 began to enter service in 1930, supplementing the O-2 in Observation Squadrons.
In 1932, at the end of the Air Corps' five year expansion plan, the O-25C was one of a number of types accepted as 'standard' observation aircraft, suitable for front line service. The later O-38B was also accepted as a 'standard' type, but none of the older O-2 models was on the list.
In February 1934 the Air Corps temporarily took over the delivery of Air Mail within the United States. The O-25C was one of the types of observation aircraft used to carry the mail.
The O-25A was the first production version of the aircraft. It was powered by a geared 600hp Curtiss V-1570-7 conqueror. Fifty aircraft were produced, with 1930 serial numbers. All fifty went to the Air Corps.
Three O-25Bs were produced. These were unarmed versions of the O-25A, with dual controls and were fur use as staff transports.
The O-25C saw the adoption of the Prestone cooled geared Curtiss V-1570-27 Conqueror engine. The radiator was moved further back and the propeller given a spinner, giving the O-25C a nicely streamlined look. Thirty were produced for the Air Corps, all with 1932 serial numbers.
Engine: Curtiss V-1570-7 conqueror
Length: 30ft 8in
Height: 10ft 10in
Empty weight: 3,400lb
Maximum take-off weight: 4,805lb
Max speed: 156.6mph
Service ceiling: 22,180ft
Armament: One fixed forward firing 0.3in machine gun, one flexibly mounted 0.3in machine gun in rear position
Bomb load: Up to four 100lb bombs