11/30/2009. Remarks by Johan Visschedijk: "Last of the Douglas observation biplanes was the O-38 series, a total of 162 were built in the following versions.

O-38: Using an airframe similar to that on aircraft of the O-25 series and fitted with a tail wheel, forty-five O-38s (30-408 to 30-419, 31-349 to 31-379, 31-406 and 31-407) were delivered to National Guard units and were powered by a 525 hp Pratt & Whitney R-1690-3 radial fitted with a Townend ring and driving a two-blade metal propeller.

O-38A: Unarmed staff transport version of the O-38 of which only one (30-407) was built for the National Guard.

O-38B: Identical to the O-38s but powered by a 525 hp Pratt & Whitney R-1690-5 radial, thirty O-38Bs (31-409 to 31-438) went to the USAAC while a further thirty-three (32-102 to 32-116, and 32-325 to 32-342) were delivered to National Guard units. One of the California National Guard's O-38Bs was experimentally powered by a 600 hp Pratt & Whitney Hornet C radial and fitted with a canopy similar to that devised for the O-38S. Two O-38Bs (31-411 and 433) were used by the Bureau of Air Commerce and received the civil registrations NC3 and NC4 respectively.

O-38C: One aircraft ordered in January 1932 by the War Department for use by the USCG (Air Corps serial 32-394 changed to USCG serial CG-9, later V-108). Identical to the O-38B but powered by a 525 hp R-1690-7 radial.

O-38D: Military designation retrospectively given to the private venture developed O-38S (unofficial designation standing for O-38 Special) which was an improved O-38 with a much wider fuselage, streamlined interplane struts and a sliding canopy covering the tandem cockpits. Powered by a 575 hp Wright R-1820-E radial enclosed in a smooth cowling, the aircraft was tested with two-blade and three-blade propellers.

Following evaluation by the Material Division at Wright Field, the O-38S (s/n 1121, civil registration X12267) was purchased October 1932 by the War Department for use by the Militia Bureau and was then officially designated O-38D (s/n 33-001), and led to the O-38E, O-38F and O-38P production versions.

O-38E: The thirty-seven O-38Es (33-002 to 33-016, and 34-001 to 34-022), with the exception of a slightly modified cowling enclosing a 625 hp Pratt & Whitney R-1690-13 radial, the use of a three-blade propeller, provision for the standard armament and an open rear canopy section, were identical to the wide-fuselage O-38S and were ordered for the National Guard. At least one O-38E operated by the 113th Observation Squadron, Indiana National Guard was fitted with twin Edo floats and had its canopy removed.

O-38F: Last War Department designation assigned to aircraft in the Douglas observation biplanes series. Eight O-38Fs (33-322 to 33-329) were delivered to the National Guard and were powered by a 625 hp Pratt & Whitney R-1690-9 radial. A new fully enclosed canopy was fitted over the cockpit of this unarmed version derived from the O-38E.

O-38P: Designation applied to six aircraft ordered in 1932 by the Aviacion Naval Peruana. Basically similar to the O-38Es of the National Guard, the O-38Ps were powered by a 640 hp Wright R-1820-F radial. The standard landing gear could be replaced by twin floats.

O-38S: Private venture (unofficial designation standing for O-38 Special), eventually became O-38D."