Designed by Ir. E. Schatzki, the Fokker D.XXI was originally designed for use by the Netherlands East Indies Army Air Service but remarkably became the leading fighter of three major European air forces. It was designed with a steel tube fuselage and wooden wings and was easily recognizable due to its fixed undercarriage yet that did not hinder its excellent maneuvrability which was one of its major traits along with its ruggedness and durability. Dutch D.XXIs were used extensively against the Germans where they shot down almost 40 Ju 52s and a number of Bf 109s before running out of ammunition on the third day; Finnish units also saw considerable action against the USSR during the Winter War and were not retired until 1951. Other users included Denmark (where they were quickly taken over by the Germans) and it was planned for production in Spain before the line was overrun by Nationalist forces.

First flight of the prototype D.XXI took place on 27 March 1936 and all production versions used the same designation although there were differences among export units. Dutch versions featured the Mercury VII or VIII engine while Finnish versions (built locall by VL) used either Mercurys or Twin Wasps. Danish versions were built by the Danish Royal Army Aircraft Factory and had Mercury VI-S engines.