In the 50s and 60s they were developing nuclear-powered jet engines so bombers could stay aloft for weeks or months. The engine itself was tested on a B36 bomber and worked pretty well, but the shielding for the crew was very heavy, and when the thing was on the ground it was very difficult to approach to work on because it was so radioactive. The airplane it was meant for was never built.
https://en.wikipedia.org/wiki/Nuclear_aircraft
Another project was to build a cruise missile powered by a nuclear ramjet, which would circle over the ocean for months on end, waiting for the order to proceed to its targets and nuke them. It could carry something like 40 nuclear warheads. This thing was pretty much the closest thing to a nuclear-powered drone anyone ever almost built. It was to be powered by a nasty unshielded reactor core so unless there was actually a war you'd never fly it over populated areas.
https://en.wikipedia.org/wiki/Project_Pluto
https://en.wikipedia.org/wiki/Supersonic_Low_Altitude_Missile
https://www.youtube.com/watch?v=qQGBBsLiP6w