 By this time, the Navy had decided the F-111 would not meet their requirements and began the development of a new dedicated fighter design, the VFAX program.In May 1966, McNamara again asked the forces to study the designs and see whether the VFAX would meet the Air Force's F-X needs.The resulting studies took 18 months and concluded that the desired features were too different; the Navy stressed loiter time and mission flexibility, while the Air Force was now looking primarily for maneuverability.