The F-4(HL) was a 1966 McDonnell proposal to the Royal Navy for a version of the F-4M with enhanced performance.
The goal of the F-4(HL) project was to obtain a better catapult performance and a lower carrier approach speed than that of the F-4M which was at that time about to enter production for the Royal Navy. The F-4(HL) was to have been powered by a pair of 20,514 lb.s.t. with afterburner Rolls-Royce RB-168-27R turbofans. It was to have had a 14-inch longer fuselage and was to have been equipped with wings with longer span (43 feet 5 1/2 inches, as compared to 38 feet 4 7/8 inches for the F-4M) and slightly less sweep (40 degrees as opposed to 45 degrees for the F-4M). The characteristic leading edge dogtooth of the F-4M was to be eliminated. The stabilators were to have had increased area as well. The air intakes were to be provided with auxiliary blow-in doors on the sides to improve the airflow to the engines at low forward airspeeds.
Assuming a project go-ahead in the late summer of 1966, a first flight of the F-4(HL) could have taken place in early 1968. In the event, the Royal Navy did not order the F-4(HL), and the project was abandoned before anything could leave the drawing board.