buffnut453
Captain
There are several sources on the internet of which I cannot validate but appear believable. Here is one however which gives some good detail. Japanese Nakajima Ki-63 Haitaka (Heinkel He 100) by comradeloganov on DeviantArt
The above source states the "design work on the Ki-61 did not begin until December 1940", after Japan had already received the 3 He 100s. One has to admit the two aircraft are uncannily similar and the noted website even indicates the Ki 63 was an Army "variant" of the He 100. And, the coincidence that the Japanese acquired 3 of the He100s makes it even more likely that some of the Ki 61 design was influenced by the Heinkel. Further, as you note, Hitachi (Nakajima) won a contract to build the aircraft which would require the transfer of technical data and all engineering drawings. Also, the Ki 61 used a licensed version of the Daimler-Benz DB 601, the same engine used by the He 100. A recent source (Zero! by Martin Caidin and Jiro Horikoshi) very strongly indicates the Japanese aircraft industry just did not have the bandwidth to crank out new designs like the U.S., Britain or Germany. One could ask, why would Japan 'reinvent the wheel' and design a whole new airplane around the DB 601 given the fact their aerospace capacity was so stretched?
Firstly, I'd never cite Caidin as a source: he was notoriously loose with facts.
I think the main problem faced by Japan's aviation industry was production capacity rather than lack of design teams or ability.
The Ki61 was started late because priority was given to the Ki60 heavyweight interceptor. Both designs were very similar,.with the 61 offering a number of design improvements that were prompted by the poor flight test performance of the Ki60.
The fact that Kawasaki was developing 2 parallel prototypes suggests that they weren't struggling for design staff. It also suggests that they weren't copying the He100; rather, they were learning for themselves based.on their own prototypes.
Finally, the Ki61 was a much larger airframe than the He100, being 3.2m longer and with a wingspan that's 2.6m greater. Those are big differences for a "copied" design.