The Pashinin I-21 was conceived in early 1939 by Mikhail Mikhaylovich Pashinin, a former employee of the Polikarpov design bureau. Pashinin sought to create a modern replacement for the aging Polikarpov I-16, incorporating combat lessons learned from the Nomonhan Incident and the Spanish Civil War. The resulting aircraft was a low-wing cantilever monoplane of mixed construction, utilizing a welded steel-tube forward fuselage and a wooden monocoque rear fuselage and wing structure. To enhance pilot visibility, the design featured an enclosed canopy, while the landing gear was retractable with a tailwheel configuration.
Technical ambition defined the I-21, particularly regarding its cooling and powerplant. The aircraft was originally intended to use the advanced Klimov M-107 liquid-cooled V-12 engine. To reduce aerodynamic drag, Pashinin implemented an experimental evaporative cooling system integrated into the wings and skin, a concept similar to the Heinkel He 100. However, the M-107 suffered from reliability issues and development delays, leading the three constructed prototypes to be powered by the Klimov M-105P piston engine, which produced 780 kW (1,050 hp).
Flight testing began in mid-1940, though sources conflict on the exact date of the first prototype's flight, citing either May 18 or July 11, 1940. State trials commenced on June 6, 1940. The aircraft's performance was hampered by the evaporative cooling system, which caused severe overheating and engine failures. While official figures claimed a maximum speed of 580 km/h, technical analyses suggest actual speeds were closer to 550 km/h. Other performance metrics included a range of 760 km, a service ceiling of 10,600 m, and a rate of climb of 21 m/s. Armament consisted of two 7.62 mm ShKAS machine guns in the wings and a centerline cannon, which was either a 20 mm ShVAK or a 23 mm BT-23 firing through the propeller hub.
Three prototypes were built in total. Prototype No. 1 revealed longitudinal stability issues; Prototype No. 2 incorporated tail modifications and completed factory tests in October 1940; Prototype No. 3, which flew in March 1941, featured revised cooling and structural changes. Despite these efforts, Soviet authorities cancelled a planned pre-series run of five aircraft. The I-21 was ultimately deemed redundant compared to the emerging Yak-1, MiG-3, and LaGG-3 fighters. No examples of the I-21 are known to survive today.