01/31/2012. Remarks by Andy Heins: "By the early 1930s, no Waco aircraft had been ordered by the military services, with the exception of a single Waco 9 ordered (s/n 26-206) by the USAAS in the Fiscal Year 1926. (Under the Project Number "P-447" it was static-tested to destruction at McCook Field.) In an effort to remedy this situation, the Model D was introduced. A single-bay staggered biplane with a faired landing gear, the type seated pilot and observer/gunner in tandem cockpits with sliding canopies.

The Model D was unusual in that the upper wing was supported by a strut which met the fuselage in the pilot's line of vision. Landing lights, flares and blind-flying instrumentation were standard, while armament consisted of one or two 0.30 in (7.62 mm) Browning machine guns with 1,000 rounds in the lower wing, optional was a flexible rear-mount machine gun of similar caliber with 500 rounds. It could also be equipped with two USAAC A-3 bomb racks mounted in tandem on the fuselage capable of carrying five 25 lb (11 kg) bombs or two 125 lb (57 kg) bombs.

The Model D was advertised as being able to perform no less than twelve military functions; these included fighter, bomber, observation post, photographic aircraft, mailplane, ambulance, trainer and seaplane. About a dozen of the Model D were produced, but the advent of the monoplane was quickly to render the type obsolete.

In the comprehensive Waco designation system the following codes were used for the produced Model D versions: WHD, S3HD and JHD.

The first letter indicated the engine, W for Wright R-975-E3 of 450 hp, S for the Pratt & Whitney R-985 of 420 hp (the 3 in S3HD meant upgraded to 450 hp), and J for the Wright R-975-E1 of 365 hp. The second letter indicated the basic airframe, in this case the Model D, while the third letter D stood for military, while armed aircraft had the suffix A.

The D-series prototype was the unarmed (WHD) built in early 1934, it was never sold and eventually dismantled by the factory. The above shown second WHD-A (c/n 3837) was registered NC14130, this had a deviating high-set tail plane, and a single machine gun, as indicated by the bulge on the lower wing on the right side. This aircraft was also flown on floats with an additional fin below the tail.

Also in 1934 appeared the unarmed S3HD Super Sport (c/n 3814), which was registered NC14048. A second aircraft was produced with a machine gun in each wing, this sole S3HD-A example was sold to Cuba and given the s/n 23.

On November 26, 1937, two WHDs (c/n 4716 and 4717) were sold to Nicaragua and given the s/n G.N.1 and G.N.2. G.N.1 crashed en route on March 6, 1938. As a replacement the earlier mentioned NC14130 was sold to Nicaragua, initially given the s/n G.N.7, this was then changed to G.N.12. The aircraft was still listed on strength on July 7, 1950.

In 1938 the last Model D version produced was designated JHD, six were delivered to Uruguay.

Four additional construction numbers (4379 to 4382) are found in Waco records but apparently were never built."