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The Douglas O-9 was a single aircraft based on the O-2 observation aircraft, but powered by a geared Packard engine in place of the Liberty engine of the original.
The O-2 was an equal span single bay biplane that won a competitive contest for a Liberty powered observation aircraft to replace the aging DH-4s in use with the US Air Service. Two prototypes had been built, one powered by the Liberty engine and one by the Packard 1A-1500. The Liberty powered aircraft was ordered into production, but the same contract also included three more Packard powered prototypes. The O-7 used a direct drive 1A-1500. The O-8 was to have had an inverted Packard engine, but was completed with an air cooled radial instead.
The O-9 was powered by a 500ph geared Packard 1A-1500 engine, with a four bladed propeller. It was later converted to the O-2A standard by giving it a Liberty engine and night flying equipment.