09/30/2012. Remarks by Johan Visschedijk: "The two-seat Falcon series originated in 1923 as the Curtiss entry in a formal US Army fly-off competition for new observation designs powered with the wartime Liberty engine. While the development and production life of the Falcon paralleled that of the Hawk, it is one of the forgotten aeroplanes of history, due mostly to the fact that it was a workhorse two-seater and did not have the glamour of the fighters. Actually, nearly as many Falcons were built for the US Armed Forces as Hawks: 170 for the Army, 27 for the Navy, 21 civil, plus undetermined export models. In 1933, the venerable Falcon was unique in being the only 1923 design still in production in the US.

Curtiss Falcons were procured for the US Army under eleven different designations using six different power plants, while the Navy models had two designations and two power plants. Export models added a seventh power option. The original Curtiss model designation was L-113, changed to Model 35 under the 1935 system (38, 44, 45, and 72 for later versions; some experimental variants were overlooked). The Army assigned the designation XO-1 to the prototype.

The Curtiss design took second place to the Douglas XO-2 in the observation design competition which took place in 1924, but the set back was only temporary. The Army decided that the ubiquitous Liberty was no longer suited to first-line military aeroplanes, so it scheduled a 1925 contest for observation types to be powered with the new Packard 1A-1500, a 510 hp twelve-cylinder V-engine very similar to the contemporary Curtiss D-12 (V-1150). All that Curtiss had to do was change engines in the XO-1. This time, Curtiss won and received an order for ten O-1s to be powered with the D-12 since the Army rejected the Packard at that time.

The Falcon had unique fuselage construction for the time- aluminum tubing bolted and riveted together with steel tie-rod bracing. The wings were wooden-framed with a wire trailing edge and the new Clark-Y airfoil. The center section was placed well forward for improved pilot access and visibility, making it necessary to sweep the upper wing panels back 9° to achieve balance. This became the Falcon's principal recognition feature.

Armament followed the traditional WW I practice, with a single 0.30 in (7.62 mm) Browning machine gun for the pilot and twin Lewis guns on a Scarff ring around the rear cockpit. Bomb racks could be fitted under the lower wing. Armament improvements are covered in the following O-1 series descriptions.

XO-1 (Model 37). On this model alone, the fin/rudder combination resembled the PW-8s. As flown in the 1924 competition, the XO-1 (s/n 23-1252) was without armament but performance figures were corrected for the weight differential. No change of Curtiss model designation was involved in the change from Liberty to Packard engine in the XO-1 for 1925 Observation aircraft competition.

O-1 (Model 37A). Ten production O-1s (s/n 25-325 to 25-334) were ordered as a result of the 1925 competition. The major difference from the prototype was the use of the Curtiss D-12 engine in place of the Packard and a revision of the vertical tail surfaces to increase the fin area and decrease the rudder balance area. Gross weight was increased from 3,857 lb (1,750 kg) to 4,165 lb (1,889 kg); maximum speed decreased from 154 mph (247.8 kmh) to 144 mph (231.7 kmh), and service ceiling dropped from 23,400 ft (7,132 m) to 17,500 ft (5,334 m). Range was increased by use of a 56 gal (212 l) auxiliary fuel tank under the belly. The first O-1 was evaluated in 1926 as a possible two-seat pursuit type. O-1s converted to other designations: s/n 25-325 to O-1 Special, an unarmed transport for VIPs, in 1926; s/n 25-331 and 25-332 to respective1y XO-13A and XO-13 racers, in 1927; 25-333 completed as O-1A.

O-1A. The ninth O-1, s/n 25-333, was completed with a Liberty engine and improved rear cockpit appointments at USAAS direction. The lines were altered by deepening the aft fuselage to form a permanent fairing behind a 56 gal (212 l) auxiliary fuel tank between the rear landing gear struts. There was no change in Curtiss designation. Although 223 lb (101 kg) heavier than the O-1, the O-1A was 4 mph (6.43 kmh) faster. It was surveyed 10 October, 1930.

O-1B (Model 37B). The first major production O-1 variant; 45 (s/n 27-243 to 27-287) were ordered in 1927. Improvements included wheel brakes, a droppable 56 gal (212 l) belly tank, and provisions for dumping the fuel in the 113 gal (427 l) main tank. An additional O-1B resulted from the conversion of one O-11 (s/n 28-207) to YO-13D and then to O-1B. Weights and performance were very close to those of the O-1. Five O-1Bs were converted to other designations: s/n 27-243 to A-3 and back to O-1B; s/n 27-244 to XA-4; s/n 27-263 to XO-18 and back to O-1B; s/n 27-264, 27-266 and 27-268 to O-1C (27-266 was reconverted to O-1B).

O-1C. Four O-1Bs (s/n 27-264, 27-266 to 27-268) were modified in 1927 in the manner of the O-1 Special to serve as personal transports under the designation of O-1C for Presidential Cabinet officers and USAAC general officers. Armament was removed, a baggage compartment with external starboard side door was added, and the rear seat was widened. As stated before, s/n 27-266 was reconverted to O-1B, but later it was converted to O-13B.

O-1D. This designation, to have been assigned to an improved O-1B with Curtiss V-1150F engine, was not used.

O-1E (Model 37I). The 41 O-1Es (s/n 29-282 to 29-322) were improved O-1Bs with V-1150E engines. Refinements included horn-balanced elevators, Frise ailerons, oleo-pneumatic shock absorbers, E-4 gun synchronizer system, and a 36 gal (136 l) belly tank. S/n 29-296 was fitted at the factory with a pilot's cockpit enclosure. Gross weight was increased to 4,532 lb (2,056 kg), maximum speed reduced to 140.8 mph (226.59 kmh) and the service ceiling to 15,300 ft (4,663 m). An additional O-1E resulted from the conversion of A-3B with s/n 30-1. O-1Es converted to other designations: s/n 29-288 to O-1F; s/n 29-295 to XBT-4, XO-1G, and Y1O-1G; s/n 29-319 to 29-321 to YO-13C and O-13C; s/n 29-322 to XO-26, Y1O-26.

O-1F (Model 37J). One O-1E, 29-288, was modified along the lines of the O-1Cs by removal of the armament, addition of a baggage compartment, and a more comfortable rear cockpit. Original power plant was the V-1150F engine, later changed to a V-1150EM.

XO-1G (Model 38). The XO-1G was a doubly converted aeroplane, having been built as O-1E (s/n 29-295) and then converted to XBT-4 for an Army Basic Trainer competition. Modification as a new observation aircraft prototype involved enough changes to justify a new Curtiss model number. Most noticeable were revision of the rear cockpit and change to a single post-mounted machine gun, cut-out of the lower rudder to accommodate a new steerable tail wheel located well aft of the original tail skid position, and completely redesigned horizontal tail surfaces. A cutout was added to the wing above the pilot. After delivery to the USAAC, this XO-1G became Y1O-1G and eventually O-1G.

Y1O-1G. The XO-1G (s/n 29-295) transferred from experimental to Service test status, and eventually became O-1G with V-1150EM engine; destroyed in a crash on June 6, 1932.

O-1G. 30 production versions (s/n 31-472 to 31-501) of the XO/Y1O-1G were ordered in 1931, and had a redesigned 'pilot's instrument panel and a new gunner's seat. Refinement of the design increased maximum speed to 143 mph (230 kmh) and the service ceiling to 17,000 ft (5,181 m) in spite of a gross weight increase to 4,426 lb (2,008 kg)."