The Jason Whitely post has the vid with a telling perspective.It looks like to me the kingcobra pilot got out of position. Either turned too late or was flying formation off the other fighters and went wide. Trying to correct he was looking at ground cues, or looking at the other fighters that he went wide on in the formation. So their relative positions and configuration of fighter aircraft, his view of the B-17 was blocked, so if the pilot looked back for other aircraft he never saw it. He also didn't catch he had started to descend in the turn. This makes sense to me.
It is "speculation" based of what little information we have. There will be more info and the final analysis could be very different.
#BREAKING: New angle of the mid-air collision obtained by @WFAA shows B-17 and other aircraft flying formations at #WingsOverDallas at 1:21p today, when it was hit by a P-63 and fell to the ground over the airfield at Dallas Executive Airport (RBD).
There were four aircraft under the P-63s' six as the P-63 banked into the B-17 from slightly above and to the B-17s' left on an intercept course. The only other single aircraft was behind the P-63. The first 4 had full view of the P-63s' belly since he was in a bank away.
With that course he had full view of the B-17 but was trying a risky maneuver going too low & fast and realized it too late.
After looking at 6 vids that's my perspective. The bottom-line is all involved are deceased, and condolences to the families & the CAF.