The Dewoitine D550, very close in its design of the D520, but more compact, had a very neat aerodynamics. It is equipped for example with a retractable stand and its weight has been considerably reduced. In addition, Dewoitine has revised its design to optimize manufacturing time from 61,000 hours to 36,000 hours.

The prototype made its first flight on 23 June 1939 in Toulouse with Marcel Doret in command. Initially powered by a Hispano-Suiza 12Ycrs 860hp, the aircraft was immediately upgraded and the aircraft, which was subsequently equipped with a Hispano-Suiza 12Y51 1,000 hp engine, would reach 702 km/h on November 22, 1939 and is able to climb to 6000m in only 4 min 25 sec. Marcel Doret is totally conquered by the D550.

However, on the Rhine, the Messerschmitt 209V1 has just landed the world record of level flight at 755 km / h. The D550 can no longer claim the title with a 1000hp engine against Messerschmitt's 1775ch ..

In October 1939, Sté Dewoitine proposed to the Ministry of the Air a militarized version of the D550: the D550-C1 project. It was the test pilot Michel Detroyat who was in charge of the evaluation of the aircraft. The aircraft has excellent flight performance and high performance. Only a few reserves are emitted on the reduced bound visibility and the landing speed too high.

The D550-C1 was changed to D551. The design of the D551 takes advantage of the D550's production time optimization, the wingspan is increased to reduce wing load, increased range with larger tanks adoption and improved visibility with a redesigned windshield. The weight remains contained, less than 2200kg compared with the 2650kg of a D520 and the expected level of speed was between 650 and 680 km / h with a rise time to 4 000 m of 3 min 50 sec. The arming consists of 5 machine guns of 7.5 mm including a machine-gun in the center of the propeller. This configuration will then be modified to respond to the current C1 program with 6 7.5mm machine guns in the wings and a 20mm gun in the axis of the propeller .

The first D551s were ready by the end of the 1939-1940 winters, and five of them only waited for their Hispano-Suiza 12Y51 engines in May 1940. It was not until the end of June that three of them Received at last, but no robbery took place before the Armistice. The manufacture of the sixteen aircraft in command was blocked by the occupant. In order to bypass the flight ban, the finished exemplaries were renamed D560, and classified as "Sport Aircraft". They were transferred to Saint-Martin-du-Touch in January 1941 for a first flight, but the Armistice Commission did not grant the flight authorizations ... The D560s and the D551s in the process of manufacture were all scrapped.

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