Between November 23 and December 17, 1939, the prototype D.520-01 made a series of test flights fitted with a Rolls-Royce Merlin III engine, before the latter was transferred to the D.521-01. The first production aircraft flew on October 31, 1939, and 437 had been completed at the time of the 1940 Armistice, of which 403 had been taken on charge by the Armée de l'Air. Production of the D.520 was resumed, with German authorization, in Vichy France, the first "new" fighter being flown on July 26, 1941, and a further 478 D.520s were built prior to and subsequent to the German occupation of Vichy France. Those built after May 1942 had the HS 12Y 49 engine which differed only in having a supercharger with a higher altitude rating.
The Wehrmacht captured 246 Dewoitine D.520s in Vichy France, to which were added a further 192 subsequently completed. Of these, 150 were delivered to Romania and 96 to Bulgaria, others being utilized by the Luftwaffe as fighter trainers. A further 72 D.520s were acquired by Italy's Regja Aeronautica, of which thirty were transferred to Germany in exchange for captured LeO 451 bombers."