How did the pilot(s) not see that coming?
The P-63 has a horrible cockpit view and many have been lost due to this reason alone. There was another P-63 flying just seconds before the B-17 if you look at the one video if it flying over the shopping center. The P-63 that hit the B-17 was most likely using the front P-63 (that was before the B-17) as a point of reference in its left banked turn. Once in the turn the P-63 could only see sky on its right side and could not see the horizon nor the other aircraft.
If the two P-63's were flying in formation, this accident would not have happened. But the fact it was trailing behind the other and it couldn't see the B-17.
There were 13,000+ B-17's made and I'm hearing the report that maybe only 5 are left airworthy now. The sounds of a radial engine is unique and once you hear them you know what type of bird it is.
I had the Liberty Bell fly over my house many years ago many times a day as it was giving rides to people over the Blueridge mountains. Sadly, the Liberty Bell was lost when it developed a fuel leak fire and landed in a farmer's field and the fire department would not go out in the field to extinguish the fire and the bird was a loss. But there is an effort to bring her back to airworthiness again.