09/30/2013. Remarks by Johan Visschedijk: "Essentially a refinement of the He 49 C, the He 51, a single-seat fighter designed by the Günter brothers flew for the first time in the summer of 1933. Overall dimensions and basic structure remained virtually unchanged, but in detail the new fighter differed markedly from its progenitor.

A single He 51 A prototype was followed by nine pre-series He 51 A-0 fighters for service evaluation, and the initial production model, the He 51 A-1, left the assembly line in April 1935. This was powered by a 750 hp BMW VI 7,22 liquid-cooled engine and was armed with two 0.311 in (7.9 mm) machine guns.

Production continued with the minimally modified He 51 B-1 which had provision for a drop tank, and after 150 He 51 were built by Heinkel and Arado, a further 150 He 51 Bs were built by Arado and 200 by Erla. An additional 200 were ordered from Fieseler of which the last 100 were completed as He 51 Cs.

The latter were intended primarily for close air support units operating in Spain and had racks of six 22 lb (10 kg) bombs. A total of 135 He 51s was shipped to Spain for use by the Legion Condor and the Spanish Nationalists. Forty-six of the He 51Bs were completed as twin-float fighters (He 51 B-2s). This type was withdrawn from first-line use and relegated to the Jagdfliegerschulen (fighter schools) in 1938."