An intensive series of test flights followed the type's maiden flight, these were not without setbacks. The most serious problem discovered during these was the presence of aeroelastic vibrations, leading to a series of investigative flights to resolve this condition. On 20 February 1957, during a test flight intended to explore the limits of the aircraft's speed-load envelope, difficulties led to the destruction of the first G.91 prototype. The cause of the prototype's loss was linked to problems encountered with the horizontal control system which had led to structural failure; this led to the tail assembly being subjected to rigorous testing to identify the cause of the failure. The re-engineering work that had been performed to cure the vibration problem was very extensive and resulted in the second prototype being fitted with a larger tail, a six cm (two in) higher canopy, and the addition of a ventral fin. The second prototype had other modifications, including the installation of a full armament and a more powerful Orpheus engine capable of 4,850 lbf. In July 1957, the second prototype performed its first flight; it was not immediately sent to participate in the final evaluation process. Both the third and fourth G.91 prototypes were later sent to France to continue the evaluation flights. The final selection of the competing designs was planned for late 1957. In September 1957, at the Centre d'Essais en Vol at Brétigny-sur-Orge, in France, the three rival aircraft types met for evaluation trials. During these trials, the Italian aircraft is claimed to have delivered an impressive performance. In January 1958, the Fiat G.91 was officially declared the winner of the competition. In April 1958, following a meeting of NATO defence ministers, an agreement was formed that the G.91 would be the first NATO lightweight strike fighter, and that it was to be followed in 1961 by "a developed version" of the Breguet Taon. In May 1958, a production meeting was scheduled to discuss the production of the aircraft with financial support from the United States; the Americans would provide some of the finance for the French, German and Italian aircraft, in addition to paying for the Turkish aircraft. The defence ministers reached an agreement to order 50 aircraft for each country. Given the large economic and commercial interests at stake, there was a certain amount of controversy surrounding this decision. After the loss of the first G.91 prototype, the French government preferred to pursue development of the locally designed Étendard instead. The British government similarly ignored the competition to concentrate on Hawker Hunter production for the same role.