On 6 August 1936, six of the He 51s were delivered to Spain to fight in the Spanish Civil War along with the faction of the army in revolt [REF].
This time of superiority was short lived, with the arrival of large numbers of modern aircraft from the Soviet Union, including the Polikarpov I-15 biplane and new Polikarpov I-16 monoplane,[REF] together with the Tupolev SB bomber, which was 110 km/h (70 mph) faster [REF].