The D.521 was a variant of the basic Dewoitine D.520 that would have been powered by a Rolls Royce Merlin III engine, giving 880 hp. The idea was first suggested in October 1939, but work did not begin in earnest until early 1940. This variant was developed by SNCAM. The first prototype (modified from D.520 No. 41) flew on 9 February 1940. During its seven test flights, the D.521 demonstrated a superior rate of climb and a top speed of 352 mph at altitude. After its last test flight on 23 March 1940 the D.521 prototype was returned to the D.520 standard.