[mountainlion]

The USAF experimented with a nuclear-powered bomber.

I think popular science had an article on nuclear airplanes but there was a radiation problem they could not solve.

[Army Air Corps]

One of the issues was the weight of the shielding.

[Ozark Tom]

There is a charged fission fragment reactor system in the works which sports a direct conversion kinetic to electric. This is not a Carnot limited process, therefore possibly could attain 85% efficiency. It is being developed with space application in mind, and probably is the best kW/kg design yet devised.

http://anstd.ans.org/wp-content/uploads/2015/07/5132_Tarditi-and-Scott.pdf

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20130001836.pdf

http://www.rbsp.info/rbs/PDF/nsstc10c.pdf

[JimRed]

...there was a radiation problem they could not solve.

Shielding was too heavy, unshielded was too hazardous. Maybe they've come up with lightweight shielding?