10/31/2010. Remarks by Johan Visschedijk: "For six years after the end of WW I, the USAAS relied almost exclusively upon the D.H.4B and D.H.4M for observation and light bombing duties. By 1924, however, it became apparent that procurement of a more modern type of higher performance could be deferred no longer. Accordingly, two series of trials were arranged at McCook Field to select a new observation aircraft; the first, late in 1924, was for aircraft powered by the Liberty engine while the second, early in 1925, was for those with the Packard IA-1500 engine.
In the 1924 evaluation, in which eleven prototypes competed, the Douglas XO-2 was declared winner, while the Curtiss XO-1 was among the unsuccessful types. The latter was forthwith re-engined with the Packard IA-1500 for the 1925 trials, which it won, however, the Packard engine failed to live up to expectation, and the production O-1, of which the Army ordered ten in 1925, had the lower-powered 435 hp Curtiss V-1150 (D-12) engine and consequently lower performance. This engine became the standard power plant for all sub-sequent O-1 variants except the O-1A (25-333), an O-1 conversion which had the 420 hp Liberty V-1650-3 and a deeper fuselage.
The 1927 production model of the Falcon, the O-1B with V-1150-5 engine, had several refinements including wheel brakes and fuel dumping provision. The 25 D-12-powered O-1Bs were armed with four 0.30 inch (7.62 mm) machineguns, two were installed in the nose and two on the rear cockpit Scarff ring. The pictured aircraft went to Wright Field where it was tested under the Project Number P-532."