Tragic. Ugly. The little plane caused the wings of the B-17 to separate from the fuselage.
Looks like he flew through the fuselage just behind the wings, the plane separated into the front half, the wings are the part forward, and the tail section. The smaller plane seems to have just disintegrated, I can't see large segments in the debris. The pilot of the P-63 was probably (hopefully) killed instantly, and the drivers of the B-17 probably were pulling back hard on the yoke, and hit the ground before they fully understood what had happened.