From the videos I've seen and the ADS-B hits I could find, it looks like the P-63 was supposed to be joining up with another group of fighters ahead and to his left. Went way wide, and hit the B-17 from behind.
Poor guys never knew what hit them. P-63 likely didn't until it was too late, belly to the 17, not sure how good/bad the visibility below and forward is on the 63, but he was likely looking at the other fighters he was trying to join.
Something like a 70 knot speed differential.