Check out for some interesting but never realized aircraft.
On one hand, I'd love to run a game set in 1947 where you're playing OSS agents sabotaging the latest ballistic missile developments. On the other, I really don't want to think about what would happen if that regime continued longer than it did.
Something similar to Iron Sky happens in the Manga/OVA - albeit in that case, they're Nazi Vampires lead by a cyborg backed by a werewolf lieutenant, invading England with zeppelins and getting killed in droves by Dracula.