In 1947, Dassault embarked on the development of an all-French fighter. Inspired by American aircraft, Dassault began building the Ouragan, which used a very thin wing, like the Lockheed P-80 Shooting Star and a basic configuration comparable to the North American F-86 Sabre.
The success of the Ouragan led to the development of the Mystère MD.452 prototype, F-WFUU, c/n 01. It was powered by a Rolls-Royce Nene turbojet engine. The first prototype Mystère I was essentially an Ouragan with a 30-degree swept wing and modified tail surfaces.
On 23 February 1951, Chief test pilot Konstantin Wladimir (“Kostia”) Rozanoff made the first flight of the Mystère MD.452 prototype, F-WFUU, c/n 01 at Istres, France. Rozanoff believed, though it was not confirmed, that he had broken the Sound Barrier.
From 1954 to 1957, a series of prototypes were built before the fighter bomber was put into production as the Mystère IIC. The prototype development was not without incident.
The prototype Dassault Mystère, F-WFUU, crashed at Istres on 3 March 1953 when a wingtip fuel tank broke away and struck the airplane’s tail, killing test pilot Charles Monier.
Kostia Romanoff was killed on 3 April 1954 while demonstrating a Mystère IVB.
Sources: Wikipedia, ThisDayinAvitation by Bryan R. Swopes.