From that angle it looks like the bomber was in the smaller plane’s blind spot, the same way you can’t see the road directly under your car, the pilot couldn’t see the bomber through the floor of his cockpit.
The P-51 came from a lower altitude, giving grand visibility. Not only that, the P-51 has an easy view of everything in front of it. It was designed specifically for intercept, which is what you saw happen.
You are correct... The Bell P-63 King Cobra (it was NOT a P-51 Mustang), suffered poor visibility, and was in a left banking maneuver (in this case, aka turning final, to be aligned for a pass down the runway) that caused the pilot to be unable to see the B-17 until the last second when the tip of the bomber’s left wing tip became visible through the front windshield of the P-63. By the time the P-63 pilot was able to see that wingtip, there no time to react. Very sad situation, and I doubt very seriously that is was intentional or a suicide. Prayers go up for the families of the 5 crewmembers of the B-17, as well as the family of the pilot of the P-63...