The Heinkel HD 28 was a reconnaissance seaplane developed in Germany in the 1920s for export to Japan. It was a conventional single-bay biplane with equal-span, unstaggered wings and three cockpits in tandem. The fuselage was braced to both the upper and lower wings with a number of struts on its sides, in addition to the normal cabane struts. The rudder extended below the line of the lower fuselage, and there was a large ventral fin fitted. The rearmost cockpit incorporated a ring mount for a gunner.
A single example built by Heinkel and supplied to Aichi as a pattern aircraft, given the designation Aichi Experimental Three-seat Reconnaissance Seaplane, for possible production in Japan, but was rejected due to problems with the engine and also failure to meet performance and weight reduction targets.