So this question was inspired by the comicbook series The Manhattan Projects which is based on the science fictional possibilities of alternate Manhattan Projects but it got me wondering if there were other things that the American government was funding during the second world war that seemed as theoretical back then as splitting an atom?
Though more answers are perfectly fine, you might wanna take a glance over this thread from 2 months ago. In particular, I think /u/restricteddata's answer here provides good definitions for the nature of superweapons and how that tied into US wartime projects.