As far as inspiration, they would have gone over many of the US and other European radial designs from the 30s, and would have sought to improve upon what they liked. The same process anybody took in any a/c design, bomber, fighter, whatever. They all stole, borrowed and modified, sometimes they had to change designs because of economic factors, or any other host of things that would crop up.
Remember too that many designs on both sides of the atlantic were followed to prototype, only to to terminated.
Choices in engines and armament tended to be "off the shelf", what can we put in our design next month? they didnt have the time to design totally new engines and guns, they sometimes did of course. If you were a US fighter designer for example, in 1940, you had two main choices, Allison inline, or PW radial, for armament, .30 or .50 caliber. Unless you wanted to seriously delay your design by coming up with a totally new engine and or gun. That could take a year or better.
Some "flukey" designs include the Mosquito, made of wood only because thats the material that the government would fund for it, and as it turned out, was the material that gave it its reduced weight and excellent performance, Fw190, if for only that it got its performance from the BMW radial, that was the only one they could get for it, P-51B/D, British design, built in the USA, and then later, a British engine, Hellcat was built in response to the Zero, and so it went.
All the odd twists and turns some of them took, and any of them might never have seen combat had their governments kyboshed them...could have been P-43D vs He118 vs Wirblewind Mk. V who knows.
