The B77 was a nuclear bomb designed to match the delivery capabilities of the B-1A bomber. This included the ability to be dropped from supersonic speeds at altitudes of 60,000 feet, or in a laydown delivery at high subsonic speeds at altitudes as low as 100 feet. Meant to replace the Mk 28 and Mk 43 in the strategic role, the program was cancelled in December 1977 due to rising costs and the cancellation of the bomber it had been designed to serve. Many components of the B77 were incorporated in the B83 which was developed in its place.