Nice article on Japanese 'secret' weapons of WWII
I'd add two footnotes to this. The first is the rumor that the Japanese were toying with the idea of manned fu-go balloons. And that experiments with captured ones might explain the infamous Roswell flying saucer crash of 1947.
The second is that Unit 731, besides being some of the biggest bastards in a world crammed full of them, may also have been incompetent and completely worthless for their so-called expertise in the Cold War. Word on the street is that their research and documentation hardly reached the junior high school level for accuracy, experiment protocols, isolating variables, etc. And that American scientists following up on it found it worthless. If true, and I suspect it is, it would make their escape from the gallows all the more deplorable.