The Mirage III family grew out of French government studies began in 1952 that led in early 1953 to a specification for a lightweight, all-weather interceptor capable of climbing to 18,000 meters (59,100 ft) in six minutes and able to reach Mach 1.3 in level flight. Dassault's response to the specification was the MD.550 Mystère-Delta, a diminutive and sleek jet that was to be powered by two 9.61 kN (2,160 lbf) Armstrong Siddeley MD30R Viper afterburning turbojets, with a SEPR 66 liquid-fuel rocket engine to provide boost thrust of 4.7 kN (1,100 lbf). The aircraft had a tailless delta configuration, with 5% thickness (ratio of airfoil thickness to length) and 60° sweep.
The tailless delta configuration has a number of limitations. The lack of a horizontal stabilizer meant flaps cannot be used, resulting in a long takeoff run and a high landing speed. The delta wing itself limits maneuverability; and suffers from buffeting at low altitude, due to the large wing area and resulting low wing loading. However, the delta is a simple and pleasing design, easily built and robust, capable of high speed in a straight line, and with plenty of space in the wing for fuel storage.
The first prototype of the Mystère-Delta, without afterburning engines or rocket motor and with an unusually large vertical stabilizer, flew on 25 June 1955. After a redesign, the vertical stabilizer was reduced in size, afterburners and a rocket motor were installed, it was renamed to Mirage I. In late 1955, the prototype attained Mach 1.3 in level flight without rocket assist, and Mach 1.6 with the rocket. The small size of the Mirage I restricted its armament to a single air-to-air missile, and it was decided during flight trials that it was too small for a useful armament load. After trials, the Mirage I prototype was eventually scrapped. Dassault considered an enlarged version, the Mirage II, with a pair of Turbomeca Gabizo turbojets; however, the Mirage II remained unbuilt as it was bypassed for a more ambitious design that was 30% heavier than the Mirage I and was powered by the new SNECMA Atar afterburning turbojet with thrust of 43.2 kN (9,700 lbf). The Atar was an axial flow turbojet, derived from the German World War II BMW 003 design.
The new fighter was named the Mirage III. It incorporated the new area ruling concept, where changes to an aircraft's cross section were made as gradual as possible, resulting in the famous "wasp waist" configuration of many supersonic fighters. Like the Mirage I, the Mirage III had provision for a SEPR 841 booster rocket engine. The prototype Mirage III flew on 17 November 1956, and attained a speed of Mach 1.52 on its 10th flight. The prototype was then fitted with manually-operated intake half-cone shock diffusers, known as souris ("mice"), which were moved forward as speed increased to reduce inlet turbulence. In September 1957, the Mirage III attained a speed of Mach 1.8.
The success of the Mirage III prototype resulted in an order for 10 pre-production Mirage IIIA fighters. These were almost two meters longer than the Mirage III prototype, had a wing with 17.3% more area, a chord reduced to 4.5%, and an Atar 09B turbojet with afterburning thrust of 58.9 kN (13,200 lbf). The SEPR 841 rocket engine was retained, it was also fitted with Thomson-CSF Cyrano Ibis air intercept radar, operational avionics, and a drag chute to shorten landing roll. The first Mirage IIIA flew in May 1958, and eventually was clocked at Mach 2.2, becoming the first European aircraft to exceed Mach 2 in level flight. The tenth IIIA was rolled out in December 1959. One was fitted with a Rolls-Royce Avon 67 engine with thrust of 71.1 kN (16,000 lbf) as a test model for Australian evaluation, with the name "Mirage IIIO". This variant flew in February 1961, but the Avon powerplant was not adopted.