The Heinkel HD 56 was a reconnaissanceseaplane developed in Germany in 1929 to operate from warships of the Imperial Japanese Navy, which designated it the Type 90-1 Reconnaissance Seaplane or E3A. It was a conventional single-bay biplane with staggered wings braced by N-type interplane struts. The pilot and gunner sat in tandem, open cockpits. Heinkel designed the aircraft at the request of Aichi, in order to submit it as their entry in an IJN competition to select such an aircraft. Heinkel built a single HD 56 prototype that was evaluated against the Nakajima Type 90-2 Reconnaissance Seaplane and the Kawanishi Type 90-3 Reconnaissance Seaplane. The Heinkel design announced the winner in 1931, on the condition that Aichi would address some shortcomings, particularly a lack of range. Refined versions of the "losing" Nakajima and Kawanishi designs would eventually see production and - in the case of the Nakajima design - in far greater numbers.
Modifications to the HD 56 by Aichi included overall reductions in length and span, the replacement of the prototype's Wright Whirlwind with a locally-built Hitachi Tempu, and numerous detail changes. Flight tests were carried out at Nagoya in August 1931, and the type was accepted into service the following year. E3As were still in service aboard Sendai class cruisers at the outbreak of the Second Sino-Japanese War.