 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 Fusing Options (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 delivery.