Interesting idea. If you intend to take that path then the following sites may be of interest.
German aircraft projects
http://www.luft46.com/
Especially
this page on the Amerika Bomber because it mentions that Doctor E. Sanger, the designer, was almost kidnapped after the war by agents of the Soviet Union because Stalin liked the idea of the Amerika Bomber. You could use this as a feature of the impending conflict between Japan and Germany with Japanese agents plotting the kidnapping of the Herr Doktor.
Strange German vehicles of the 1930s-40s
http://strangevehicles.greyfalcon.us/
Plus a page claiming to show some Nazi futureworld ideas
http://futureworld.greyfalcon.us/
Japanese experimental vehicles
http://mailer.fsu.edu/~akirk/tanks/japan/japan-exp.html
Italian experimental vehicles
http://mailer.fsu.edu/~akirk/tanks/I...ntalTanks.html
And if you really want to waste an entire afternoon reading a forum thread, this one has a lot of experimental vehicles from various nations
http://www.histomil.com/viewtopic.php?f=345&t=5294