It’s not hard to lose sight during a turning rejoin. This is a fundamental formation skill taught very early in military pilot training. You remain below the aircraft you are rejoining on so if you can’t control your rate of closure, you can roll wings level and pass below the airplane you are trying to rejoin on. If you roll into more bank to try to slow your closure, you lose sight of the airplane you are closing on. The P-63 pilot in this case most likely lost sight of the B-17. If he knew it was there, he should have rolled wings level and pushed his nose over. Instead, he kept his bank until impact.
I don’t know who was flying the P-63 or what his background was. But he screwed up. Flying high performance aircraft near each other takes professional skill. It’s awesome seeing classic aircraft in flight, but the warbird community needs to knock this stuff off. There aren’t enough airplanes left to lose them to stupid mistakes.
B-17 was the “Texas Raider”, been flying airshows since the early ‘60s.. belonged to the CAF.