The Heinkel He 51B-1 was the first serial fighter of the German Luftwaffe. The first prototype, named He 51a, was designed by the Gunter brothers and first flew in the summer of 1933. The modification He 51B-1 had provisions for a drop tank and was produced since 1936. A total of 135 He 51 aircraft were shipped to Spain for use by the Legion Condor and the Spanish Nationalists as fighters and fighter/bombers. Several future aces piloted these in Spain, including A. Galland, H. Harder and H. Trautloft. He 51 aircraft were gradually transferred to Spanish Nationalist Air Force Units when they proved at a disadvantage to the Republican I-15 and I-16 fighters in air battles.