For military it has been looked into. Issue at the time was weight vs power.

Main issue being such an aircraft cannot become a unaimed bomb in the event of downing somehow. And while air may statically be the safest way to travel. Planes have had many issues. More so during the time of this research.

So only with the nuclear unit contained in a 18000ft drop saft container. Is such an idea considered viable. Subs and ships do not need that level of protection. Or have that much limitation on power to weight.