In 1931, Heinkel recruited the talented aircraft designers, Walter and Siegfried Günter, and their first major design for Heinkel was the Heinkel He 49. While this was officially an advanced trainer, in fact it was a fighter. The first prototype, the He 49a, flew in November 1932, and was followed by two further prototypes, the He 49b, with a longer fuselage, and the He 49c, with a revised engine.
The type was ordered into production for the still secret Luftwaffe as the He 51, the first pre-production aircraft flying in May 1933. Deliveries started in July of the next year.
The He 51 was a conventional single-bay biplane, with all-metal construction and fabric covering. It was powered by a glycol-cooled BMW VI engine, with an armament of two 8 mm (.323 in) machine guns mounted above the engine.
The He 51 was intended to replace the earlier Arado Ar 65, but served side-by-side with the slightly later Ar 68. The He 51 was outdated the day it entered service, and after an initial run of 150 production fighters, the design was switched into the modified He 51B, with approximately 450 built, including about 46 He 51B-2 floatplanes, and then finally a further 100 He 51C light ground-attack plane.