In September 1968, a request for proposals was released to major aerospace companies. These requirements called for single-seat fighter having a maximum take-off weight of for the air-to-air role with a maximum speed of Mach 2.5 and a thrust-to-weight ratio of nearly 1:1 at mission weight. It also called for a twin-engined arrangement, as this was believed to respond to throttle changes more rapidly and might offer commonality with the Navy's VFX program. However, details of the avionics were left largely undefined, as whether to build a larger aircraft with a powerful radar that could detect the enemy at longer ranges was not clear, or alternatively a smaller aircraft that would make detecting it more difficult for the enemy.