In the all-time history of bad ideas:

Project Pluto:

The proposed use for nuclear-powered ramjets would be to power a cruise missile, called SLAM, for Supersonic Low Altitude Missile. In order to reach ramjet speed, it would be launched from the ground by a cluster of conventional rocket boosters. Once it reached cruising altitude and was far away from populated areas the nuclear reactor would be turned on. Since nuclear power gave it almost unlimited range, the missile could cruise in circles over the ocean until ordered 'down to the deck' for its supersonic dash to targets in the Soviet Union. The SLAM as proposed would carry a payload of many nuclear weapons to be dropped on multiple targets, making the cruise missile into an unmanned bomber.

Also known as the Flying Crowbar:

Pluto's namesake was Roman mythology's ruler of the underworld -- seemingly an apt inspiration for a locomotive-size missile that would travel at near-treetop level at three times the speed of sound, tossing out hydrogen bombs as it roared overhead. Pluto's designers calculated that its shock wave alone might kill people on the ground. Then there was the problem of fallout. In addition to gamma and neutron radiation from the unshielded reactor, Pluto's nuclear ramjet would spew fission fragments out in its exhaust as it flew by. One enterprising weaponeer had a plan to turn an obvious peace-time liability into a wartime asset: he suggested flying the radioactive rocket back and forth over the Soviet Union after it had dropped its bombs.

[...]

Meanwhile, at the Pentagon, Pluto's sponsors were having second thoughts about the project. Even before it began dropping bombs on our enemies Pluto would have deafened, flattened, and irradiated our friends. The noise level on the ground as Pluto went by overhead was expected to be about 150 decibels. Ruptured eardrums, of course, would have been the least of your problems if you were unlucky enough to be underneath the unshielded reactor when it went by, literally roasting chickens in the barnyard. Pluto had begun to look like something only Goofy could love.

I can't believe this was never completed.