The He 51 originated from three prototype He 49 biplanes designed by Walter and Siegfried Gunter. Although unarmed, the He 49a, a small machine with a 750-hp BMW VI engine, had basic fighter potential, with a top speed of approximately 320 km/h (200 mph). The He 49a flew for the first time in November 1932; the second prototype, the He 49b, flew in February 1933 and had a fuselage 40 cm (15.7 in) longer, and was later converted to twin-float configuration; a third prototype, the He 49c, had a wheeled undercarriage with leg and wheel fairings.
The high performance of all these aircraft during tests prompted a fourth prototype, refined even further and lighter in weight, and this received the new designation He 51a.
Engine: 1 x 690hp BMW VI 6,0
Max take-off weight: 1950 kg / 4299 lb
Wingspan: 11.00 m / 36 ft 1 in
Length: 8.24 m / 27 ft 0 in
Height: 3.50 m / 12 ft 6 in
Wing area: 27.70sq.m / 298.16 sq ft
Max. speed: 325 km/h / 202 mph
Cruise speed: 285 km/h / 177 mph
Rate of climb: 606 m/min / 2000 ft/min
Armament: 2 x 7.9mm machine-guns