The Potez design progressed no further than a full-scale mock-up, but construction of a prototype of the Dewoitine fighter started at the Bagnères-de-Bigorre factory of the railway engine manufacturer Soulé in November 1939.
Its fuselage closely resembled that of the D.520, with the radiator moved from the ventral position of the D.520 to under the nose, to avoid spray from the aircraft's floats during take-off and landing.
[1] The inverted gull wings were entirely new, and were fitted with full-span slotted fowler flaps, which extended under the fuselage.
[2] The wings were designed to fold for storage aboard ship, with the wing-folding mechanism powered by compressed air.
[1] The prototype was completed in April 1940, but did not fly, with the French Navy now wanting carrier-based fighters,[1] ordering two prototypes of a carrier-based landplane derivative, the HD.790 and adopting the D.520 as a land-based fighter, while the Soulé factory concentrated on the D.550 derivative of the D.520.