Back in the mid 1950's, nuclear powered cruise missiles were being studied and in 1957, development was initiated as Project Pluto. The reactor for the missile was to be developed by the Lawrence Radiation Laboratory, with the ramjet being built by Marquadt. Ling-Temco-Vought was awarded the contract to develop the airframe for the missile itself, which was known as SLAM (Supersonic Low-Altitude Missile).
The mission profile called for SLAM to be launched from hardened silos, propelled by solid rocket boosters up to ramjet ignition speed. SLAM would then cruise at high altitude (35,000ft/10,700m) at Mach 4, where it's range was estimated to be more than 10,000km. This meant the missile could 'loiter' at a fail safe point until it was ordered to either continue to it's target or abort its mission. Over enemy territory it would descend to low level, using TERCOM (Terrain Contour Matching) to find its way to multiple targets.
It was envisaged that between 16 and 24 thermonuclear weapons would be carried, and ejected one by one as it flew over it's targets. The weapons would be launched on a lofted trajectory from hatches in the top of the missile to allow it a few seconds to escape the blast. The mission profile gave rise to a number of unique features. Flying at low level, the sonic boom of a 25m long aircraft flying at Mach 3 would flatten any unhardened structures, while the exhaust from the unshielded nuclear reactor would leave a trail of radioactive debris. It was estimated that the sound level on the ground could be as high as 150db. Once the weapons were exhausted, the missile itself could be used as a weapon - crashing a 'hot and dirty' reactor onto the ground would leave a very large area uninhabitable for many years.
The first flight was due in 1967, but by 1964 the project was in trouble. Intercontinental Ballistic Missiles (ICBMs) were in place which could strike the Soviet Union, and SLAM wasn't expected to be in service before 1970. Also, flight testing proved to be a problem, as it couldn't be tested over land due to the radioactive exhaust, and testing over the sea would mean crashing the missile, complete with several tons of nuclear reactor, into the ocean. Also, the whole venture was getting more and more expensive, and the project was finally cancelled in July 1964.