The further development of fighter-biplane constructions of N.N. Polikarpov became the I-15. The first prototype of I-15 (TsKB-3) with Wright “Cyclone” engine made its maiden flight in October 1934. The main characteristic feature of this little biplane was the gull-shaped upper wing. I-15 had excellent maneuverability and was the best fighter in the world in this characteristic. I-15 received the baptism of fire during the Spanish Civil War (1936-1939). Together with I-16 these aircraft, nicknamed “Chatos”, became the main Spanish Republican Aviation fighters. I-15 showed in combats substantial advantage over Nationalist He 51 and FIAT CR-32 fighters.