In one proposal, Dassault would have entered a partnership with the British Aircraft Corporation to jointly produce a Mirage IV variant for the Royal Air Force and potentially for other export customers, but this project did not come to fruition.

In May 1956, the Guy Mollet government drew up a specification for an aerially-refuelable supersonic bomber capable of carrying a 3 tonne, 5.2-metre-long nuclear bomb 2,000 km (without aerial refuelling).

[2] According to aviation authors Bill Gunston and Peter Gilchrist, the specification's inclusion of supersonic speed was "surprising" to many at the time.

[5] It was considerably more advanced than the Mirage III, incorporating new features such as machined and chem-milled planks, tapered sheets, a small amount of titanium, and integral fuel tanks in many locations including the leading portion of the tailfin.

[6][7][8] On 19 September 1960, René Bigand (replacing Glavany as test pilot) increased the world record for speed on a 1000-kilometre closed circuit to 1,822 km/h (1,132 mph) around Paris and the Melun base.

The Mirage IV 01 prototype underwent minor modifications during testing in the autumn of 1959, most noticeably, the tail was enlarged (slight reduction in height, large increase in chord).

[10] In order to increase range, studies were made of a significantly larger Mirage IVB design, powered by two Snecma license-built Pratt & Whitney J75 engines and having a wing area of 120 m² (1,290 sq ft) compared to 70 m² (750 sq ft) of the prototype IV, as well as a speed of Mach 2.4 and a gross weight of 64,000 kilograms (140,000 lb).

[11] The Mirage IVB proposal had been instigated as a response to interest by de Gaulle in ensuring that two-way (including the aircraft's return to France) strike missions could be flown.

[8] However, development of the aircraft was ultimately cancelled in July 1959 due to the greater cost involved, a decision having been taken to rely upon aerial refueling instead also being a factor.

[7] Manufacture of both the prototypes and subsequent production aircraft was often hindered by an explicit requirement that there would be no reliance upon foreign suppliers to maintain France's nuclear capabilities; due to this, the Mirage IV initially lacked an inertial navigation system as French industry could not yet produce this device.

Although Dassault had designed the Mirage IV for the low-level flight role right from the start,[16] the final batch of 12 aircraft ordered in November 1964 differed from the earlier aircraft in several areas, including the flight controls, avionics, and structural details, for the purpose of providing improved low-level performance.

However the operational requirement still existed, so in response, Hawker Siddeley offered the Buccaneer S.2, the Americans the General Dynamics F-111K, while, in July 1965, Dassault and British Aircraft Corporation (BAC) jointly proposed a modified Mirage IV.

[21][22] The Dassault/BAC aircraft, known as the Mirage IV* or Mirage IVS (S for Spey) would be re-engined with more powerful Rolls-Royce Spey turbofan engines with a total of 185 kilonewtons (41,700 lbf), larger (fuselage depth increased by 7.6 centimetres (3 in), had an approximately 0.61 metres (2 ft) forward fuselage extension, and was to weigh 36,000 kilograms (80,000 lb)), and use avionics planned for the TSR-2, although BAC preferred the French Antilope radar.

[26] Some aviation journalists claim that the rejection of the Mirage IV* may have dampened French interest in joint Anglo-French cooperation.

[32] Other avionics elements were provided by Dassault itself and SFENA; one of the only major subsystems not of French origin onboard was the Marconi-built AD.2300 doppler radar.

The outer pylons typically carried ECM and chaff/flare dispenser pods to supplement the internal jamming and countermeasures systems.

The early Mirage IVA had a fuselage recess under the engines which could hold a single AN-11 or AN-22 nuclear weapon of 60 kt yield.

[37] During the 1980s, a total of 18 Mirage IVs were retrofitted with a centreline pylon and associated equipment to carry and launch the nuclear Air-Sol Moyenne Portée (ASMP) stand-off missile.

[38] The Mirage IVA could theoretically carry up to six large conventional bombs at the cost of drop tanks and ECM pods; such armament was rarely fitted in practice.

A justification of the Mirage IV given by Armée de l'air Brigadier General Pierre Marie Gallois, an architect of the French nuclear deterrent, was that: "France is not a prize worthy of ten Russian cities".

[48] Even so, some sources state that some of the mission profiles envisioned were actually one-way, with the crew having no chance of returning after bombing a Soviet city.

[47] Both flight and ground crews received training principally by Strategic Air Forces Command 328, stationed at Bordeaux.

[50] Initially, the basic attack flight profile was "high-high-high" at a speed of Mach 1.85, engaging targets up to a maximum radius of 3,500 km (2,175 mi).

In the late 1960s, when the threat of surface-to-air missile defences made high-altitude flight too hazardous, the Mirage IVA was modified for low-altitude penetration.

[17] By 1964, Mirage IVAs were conducting training penetration runs at an altitude of 200 ft, without the assistance of terrain-following radar, which subjected pilots to considerable workload and those on board to high levels of turbulence.

[48] To improve survivability, the French Air Force began dispersing Mirage IVs to pre-prepared rough strips during the 1960s; while the use of hardened bunkers had been assessed, it was found to be financially impractical.

[48] By the 1970s, it had become clear that vulnerability of the Mirage IV to air defences, even while flying at low altitudes, had made the delivery of gravity bombs such as the AN-11 or AN-22 impractical.

[51][52] In 1973, it was reported that a force of 40 Mirage IVs would continue to perform as a part of France's nuclear deterrent until the 1980s, and that steady improvements were to be undertaken.

ES 1/91 Gascogne's surviving Mirage IVPs were retired in 2005 and are conserved and stored at the Centre d'Instruction Forces Aériennes Stratégiques (CIFAS) at Bordeaux Mérignac.