Project Pluto!

Nope, it has nothing to do with either the former planet or Mickey's dog.

Instead, it was a plan for a nuclear-powered nuclear cruise missile that
actually got as far as engine testing in the early 1960s.

http://www.merkle.com/pluto/pluto.html

We're talking a half-gigawatt nuclear rocket that could wait in the sky
circling the Pacific Ocean for its orders, then fly at tree top level at
Mach 3 to deliver perhaps a dozen hydrogen bombs, all the while
spreading radiation and other destruction in its flight path.

One project engineer even considered that after Pluto dropped off its
payload, it could fly back and forth over the Soviet Union, spraying
even more radiation with its nuclear engine for as long as possible.