Donald N. Frey was the head engineer for the T-5 project—supervising the overall development of the car in a record 18 monthswhile Iacocca himself championed the project as Ford Division general manager. The T-5 prototype was a two-seat, mid-mounted engine roadster. This vehicle employed the German Ford Taunus V4 engine.
It was claimed that the decision to abandon the two-seat design was in part due to the increase in sales the Thunderbird had seen when enlarged from a two-seater to a 2+2 in 1958. Thus, a four-seat car with full space for the front bucket seats, as originally planned, and a rear bench seat with significantly less space than was common at the time, were standard. A “Fastback 2+2”, first manufactured on August 17, 1964, enclosed the trunk space under a sweeping exterior line similar to the second series Corvette Sting Ray and European sports cars such as the Jaguar E-Type coupe.