In 1964 the Air Force issued a contract to TRW to consider the problem of ICBM survivability under the name "Golden Arrow", looking for ways to make their own missiles as invulnerable as the Navy's. Ideally, the US ICBM fleet should be able to ride out an attack and then launch a response, which would require as many as 1,000 warheads, one for each Soviet ICBM silo. Not only did they have to survive, but they had to have the ability to be quickly retargeted so the surviving US missiles could be aimed only at the remaining Soviet missiles, not empty silos.
