The Polikarpov I-16 was a Soviet fighter aircraft of revolutionary design; it was the world's first cantilever-winged monoplane fighter with retractable landing gear. 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. The biggest complaint in service was the light armament of only two 7.62mm (0.30 cal) machine guns. This was urgently addressed with Type 6 which added a third ShKAS in the bottom of the fuselage.