Designed by Ernst Zindel, the Junkers J 21 (also known by the sales designations T 21 and H 21) was an all-metal, two-seat reconnaissance monoplane. The aircraft first flew on 12 June 1923 at Dessau, Germany. Its design featured a parasol-wing layout and two open cockpits arranged in tandem, with the pilot positioned in front of the observer/gunner. The airframe utilized a fixed, tailskid undercarriage and was constructed entirely of metal, a hallmark of Junkers Flugzeugwerke AG that contrasted with the wood-and-fabric construction common in the early 1920s.
While prototypes were developed in Dessau, series production was carried out in the Soviet Union at the Junkers plant at Fili, located near Moscow. Approximately 120 aircraft were produced during the mid-1920s. The prototype version was powered by a BMW IIIa six-cylinder, water-cooled inline piston engine producing 138 kW (185 hp), which allowed for a maximum speed of 217 km/h and a range of 750 km. The production version was upgraded to a more powerful BMW IV water-cooled inline engine. For combat operations, production aircraft were equipped with machine guns for both the pilot and the observer.
Operationally, the J 21 was utilized by the Soviet Air Force to replace the World War I-era de Havilland DH-9A. However, the aircraft's performance was only marginally superior to the DH-9A, leading to its rapid replacement by the Polikarpov R-1, a Soviet-licensed derivative of the DH-9A. Additionally, the J 21 was evaluated by the Reichswehr at the clandestine training school in Lipetsk, Soviet Union; however, it was rejected in favor of the Heinkel HD 17 due to poor performance.
The J 21 served as a platform for further experimentation. This included the J 22 (T 22), a single-seat fighter prototype that first flew on 30 November 1923, and the J 28, a two-seat fighter variant powered by a 400 hp BMW VI engine. Neither variant entered production. Today, no complete airframes of the J 21 are known to survive in public museum collections.