Prop wash from a bigger airplane causes turbulence behind it and a little to the sides of them and may lead to loss of the control by the trailing aircraft. The wingtip vortices turn into circular air forming the vortices, NOT GOOD
The P63 looks like it was flying a lot faster than the B17 when it hit the B17 so maybe he did lose control
I knew medical Dr who lost his life due to getting in too close while landing behind a medium size passenger commuter jet landing at Ontario, CA years ago in his small single engine Bonanza. The standing rule is two minutes between aircraft landing on the same runway, any closer can cause turbulence and problems on maintaining control of the smaller aircraft.
Larger airplanes have to follow the same rules also.
I have experience it when following a friend of mine in a King Air SP350 in my much smaller Piper Comanche single engine landing in San Fernando Valley at Van Nuys airport, I let my plane get lower than him and got into his turbulence when crossing behind him to get line up with the runway, I climb up to be above him and made sure I landed further down the runway than him to avoid more turbulence , Once learn you make sure it doesn't happen again, I had been taught about wake turbulence and following distances but didn't do it correctly, he was landing from the west landing to the south and I had approach the airport from the east and was landing on the same runway less than 2 minutes behind him I got lucky, thanks be to God