fuel than the Mirage III. The aircraft's aerodynamic features were very similar to the III's but required an entirely new structure and layout. This prototype was long, had an wingspan, of wing area, and weighed approximately . 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. The 01 was an experimental prototype built to explore and solve the problems stemming from prolonged supersonic flight. At the time, no aircraft had been designed to cruise at over Mach 1.8 for long periods of time and there were sizable technological and operational uncertainties. Weapon-related issues were another issue. Building the 01 in Dassault's Saint-Cloud plant near Paris took 18 months. In late 1958, the aircraft was transferred to the Melun-Villaroche flight testing area for finishing touches and ground tests. On 17 June 1959, French General Roland Glavany, on a five-year leave from the French Air Force since 1954, took the 01 for its maiden flight. 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. Flight 138, on 23 September, corroborated the initial performance and pushed the record on a 500 km closed circuit to an average of 1,972 km/h (1,225 mph), flying between Mach 2.08 and Mach 2.14. 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). Production 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 . 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. 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