In July 680 B-29 Bombers flew over Japan, 3 were shot down.
The Japanese weren’t even trying to attack individual aircraft by then, only sending their fighters up against massed formations of American Bombers where there was a possibility of them getting in amongst the bombers before being pounced on by the P-51 escorts.
I am not saying it is impossible. I’m just saying the Enola Gay would have had to be very fucking unlucky.
Ok, so they shoot down the Enola Gay. In all likelihood the B-29 would have limped back towards Tinian or tried for Iwo Jima, crashing into the sea. In that case the first successful Atomic Bombing mission is carried out by B-29 Bockscar three days later.
If Enola Gay crashed on Japanese soil, they would have perhaps attempted to recover the Aluminium from its airframe, they would have just left the engines and the other strange wreckage as it wouldn’t have had any value to their war effort.
You weren’t expecting an undamaged Atomic Bomb to be sitting in the wreckage were you?