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The Siemens-Schuckert Werke SSW D.II was ordered as a test bed for the new Siemens-Halske S.H. III rotary engine. It followed on from the SSW D.I, which was basically a copy of the French Nieuport 17 but with a Siemens engine. The D.II was a more advanced design, produced to take advantage of the increased power expected to come from the S.H.III engine. Three prototypes were ordered in November 1916, and the first two airframes were completed by January 1917.
The SSW D.II was a small, stocky, barrel-like aircraft. It was built around four longerons, connected by circular bulkheads. These were linked by formers, and the entire frame was covered with three-ply plywood.
The new Siemens-Halske S.H. III engine was an unusual rotary engine, in which the crankshaft revolved in one direction and the cylinders in the other direction. Both rotated at 900 rpm, for an effective overall engine speed of 1,800 rpm. In a normal rotary engine the crankshaft was fixed and the crankcase and cylinders revolved around it, normally at around 1,200-1,500 rpm. Standard rotary engines were very powerful, but with everything spinning in the same direction they tended to impart some of that spin to their aircraft, making them quite tricky to handle. In the S.H. III that effect was greatly reduced.
The S.H.III ran at a high compression ratio, which improved its performance at higher altitudes. It was also unusual for a rotary engine in that it had a proper throttle, allowing for more sensitive control of power. Other rotary engines were either fully on or fully off, or were controlled by briefly cutting off power to some of the cylinders, a potentially dangerous method.
The new engine did have a number of problems. The air cooled cylinders were bigger and more powerful than in earlier engines, but they were also moving significantly more slowly, so they weren't cooled as effectively. As with many unusual new engines work on the S.H.III was slow, and they weren't available for use in the new SSW D.II until June 1917.
By that time the airframes were ready, and so the aircraft was soon ready for tests. In August it demonstrated a remarkable rate of climb, reaching 5,000m (16,400ft) in 15.5 minutes and 7,000m (22,965ft) in 35.5 minutes. Level speed was less impressive, but the climb rate was good enough to produce an order for three new prototypes. These were ordered as the D IIc kurz (short), D IIc lang (long) and D IIe. The first two of these aircraft were completed in October 1917, and at that point were redesignated as SSW D IIIs. The redesignated D IIIc kurz began test flights on 22 October 1917, and the D III c lang followed on 15 November.
The successful trials were followed by a pre-production order for twenty D III aircraft, based on the D IIc kurz, and in February 1918 by an order for thirty S.S.W. D III production aircraft.
The D IIc kurz had a wingspan of 27ft 10.75in and a wing area of 209sq ft. The D IIc lang had a wingspan of 29ft 6 3/8in, but the upper wing had a reduced chord so the wing area was only 195sq ft. The D IIc kurz was more similar to the D III, while the lang resembled the S.S.W. D IV.
The D IIe was another experimental design. It used dural girder spars in the wings, which were linked by I struts. The aim was to eliminate the interplane bracing cables, which kept the wings rigid but added weight and drag. Unfortunately the new wings weren't rigid enough as first built, and had to have cables added. This made it rather pointless, and so it was rebuilt to turn it into a standard S.S.W. D IV. The aircraft then went to the front, where it served with Geschwader II.