Page, head of the Saint Louis Airplane Division of Curtiss-Wright, decided to develop a fighter aircraft based on Carl W. Scott's two seater Model 19.

Page's concept was a lightweight fighter interceptor with as high a rate of climb as possible in order to allow bomber formations to be attacked with minimal warning.

[citation needed] In May 1939, a contract was signed, with China receiving the prototype and three complete examples built by Curtiss, as well as kits for 27 more aircraft.

The main difference was a new undercarriage with inward-retracting mainwheels and a semi-retractable tail wheel which had been developed for the Curtiss-Wright CW-23 armed trainer, with other changes including hydraulically operated flaps.

After the Battle of the Netherlands, which resulted in the Dutch Army surrendering to the invading Germans on 15 May 1940, the order for the CW-21Bs (together with a number of Curtiss Model 75 fighters and Curtiss-Wright CW-22 trainers), was transferred to the government of the Dutch East Indies (now Indonesia), for the Militaire Luchtvaart van het Koninklijk Nederlands-Indisch Leger ("Military Aviation of the Royal Netherlands East Indies Army"; ML-KNIL).

The lightweight construction of the Curtiss-Wrights gave rise to structural problems, and several aircraft were grounded by cracks in the undercarriage, and were still awaiting repair when war with Japan began on 8 December 1941.

2-VLG IV claimed four aerial victories during the Netherlands East Indies campaign, but the ML-KNIL was overwhelmed by the sheer number of Japanese aircraft; almost all of its fighters were soon lost in combat or destroyed on the ground.