The D.520 was named for its prototype's impressive (for the time) maximum speed, 520 kph. It was the only plane in the Armee De L'Air with performance adequate to survive against the Luftwaffe's Me109, but very few of them had been delivered to combat squadrons when the Germans invaded France. (The principal fighter in France at the time was the M.S.406, which was so lame it isn't even included in this encyclopedia.) They continued to serve with Vichy France during the war, and flew for many years after the war as trainers.
Cool stuff for D.520 lovers: