The Mark 27 was designed by the University of California Radiation Laboratory (UCRL; now Lawrence Livermore National Laboratory) starting in the mid-1950s. The basic design concept competed with the Los Alamos Scientific Laboratory (LASL; now Los Alamos National Laboratory) design that would become the Mark 28 / B-28 nuclear bomb and W28 warhead. The Mark 27 was roughly twice as heavy as the Mark 28/B-28/W28 family of thermonuclear weapons. The Mark 27/W27 devices had a yield of versus the (later ) of the Mark 28/B-28/W28 weapons.
