The Polikarpov I-16 (Russian: Поликарпов И-16) is a Soviet single-engine single-seat fighter aircraft of revolutionary design; it was the world's first low-wing cantilever monoplane fighter with retractable landing gear to attain operational status and as such "introduced a new vogue in fighter design". The I-16 was introduced in the mid-1930s and formed the backbone of the Soviet Air Force at the beginning of World War II. Nicknamed "Ishak" or "Ishachok".