The B77 was a nuclear bomb designed in 1974 to match the delivery capabilities of the B-1A bomber.
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 including its already tested physics package (the actual bomb core) were incorporated in the B83 which was developed in its place.
The specifications for the B77 required Full Fuzing Option (FUFO) and the ability for a low altitude, transonic laydown delivery, as well as a free fall from supersonic speeds and altitudes of 60,000 feet (18,000 m) delivery.
This combination would actually lift the bomb from a drop altitude of 100 to 300 feet (30 to 91 m) for main parachute opening.