The next Falcon contract on June 19, 1929, ordered the A-3B and O-1E versions using the V-1150-5, balanced (Frise) ailerons and elevators, and a .30-caliber M1922 Browning flexible gun in the rear, instead of the drum-fed Lewis guns. Seventy-eight A-3Bs were built on two contracts, the first tested in April 1930 and armed with five Browning guns, 2,700 rounds, and two 116-pound bombs on wing racks. Attack Falcons equipped all four of the Air Corps ground-attack squadrons; the 8th, 13th, and 90th of the 3rd Attack Group at Fort Crockett, Texas, and the 26th Attack Squadron in Hawaii.
Similar to the A-3B, but armed with just one fixed and one flexible Browning with 500 rounds per gun, was the O-lE appearing in December 1929. Curtiss built 35 O-lE, one unarmed O-lF transport, and in October 1930, one XO-lG (later Y1O-lG) introducing such refinements as a new rear gun post mount allowing more cockpit room and a recess to fold the gun away, wheel pants, and tail wheel. The latter was adopted and retrofitted to earlier Falcons, but the pants proved unsuitable for service use.
Thirty O-lG Falcons were ordered January 17, 1931 and delivery began in May 1931. Their equipment included the usual two Brownings, camera, and a 109-pound radio (SCR-134) with a 30-mile range with voice and 150 miles with Morse code.
Since each Army observation squadron was assigned to work on its own with particular infantry corps and divisions, group organization had been limited to the 9th Observation Group formed with the three squadrons (1st, 5th and 99th) based at Mitchel Field, New York. That group was equipped with Curtiss Falcons, which also went to the 12th and 16th Observation squadrons. In October 1930, the 12th Observation Group was activated at Brooks Field, Texas, to control O-l, O-2 and O-l9 squadrons in the western states.
The Falcon series also included several examples testing new engines. First was the air-cooled-Pratt & Whitney R-1340-1 flown May 21, 1928, on the XO-12, which looked like the XA-4 and had originally been an O-l1 airframe. The Curtiss V-1570-1 Conqueror was introduced in September 1927 on the XO-13 and XO-13A conversions of O-1 airframes, with the XO-13A having wing radiators good for racing, but not service use. There was also an O-13B transport converted from an O-1C, and three YO-13C Falcons with the usual tunnel radiators followed the O-1E off the Curtiss line in August 1930.
The Conqueror engine was adapted for Prestone cooling and used on the XO-16 ordered December 28, 1927, and completed in May 1930. The usual two Brownings were carried, but the rear M1922 gun was on the new fold-away post mount.