Rocketdyne developed the F-1 and the E-1 to meet a 1955 U.S. Air Force requirement for a very large rocket engine. . . . The Air Force eventually halted development of the F-1 because of a lack of requirement for such a large engine. [0]

The fifties were such a crazy time where the USAF would commission wild things which in development were superseded or mooted by other developments, leaving the commissioned object as a cast-off vision of a future that didn't happen. Maybe the high point of this was the XB-70 Valkyrie. [1]

Today is the first day that I put two and two together on North American Aviation making both the XB-70 and the B-1: To take maximum advantage of [compression lift], they redesigned the underside of the aircraft to feature a large triangular intake area far forward of the engines, better positioning the shock in relation to the wing.

To bring things full circle, in 1955 NAA spun off its rocket division, which became Rocketdyne, the maker of the F-1 among other engineering marvels.

0. https://en.wikipedia.org/wiki/Rocketdyne_F-1

1. https://en.wikipedia.org/wiki/North_American_XB-70_Valkyrie