That doesn't explain why the economy improved, from building bombs, dropping bombs, or shooting Germans. No, I don't think having Hitler running our country would have been beneficial, but that didn't happen, and the economy improved.
There was no direct economic benefit from shooting Germans or dropping bombs. We didn't receive $15 dollars for every German plane that we shot down, or building we destroyed. Could you imagine if the government would have spent a bunch of money on things that had a return on investment, even a small return on investment? I can, because Eisenhower did it in the 50's, with roads, schools, and bridges.
We got all that economic growth from paying people to make bombs, then paying people to drop bombs? That economic growth came, in part, from artificially inflating the labor market. Something that we could really use today.
So, my broader point is, we don't need WWIII to spur economic growth. But infrastructure spending could do, and has done, the same thing. Improving mass transit, rural broadband, building windmills, ect., there are so many things we could do that would have a higher economic return than blowing up a German tank.