So lets dumb this down so maybe folks will get it.
Aces High at best represents a tactical airwar. The B29 was a long range strategic bomber. It did not fly in support of ground troops, it supported no invasions, didn't go after airfields etc. It was basically used to burn Japanese cities to the ground, and it did it well.
We don't have a strategic aspect to Aces High and last I saw, burning cities to the ground wasn't an option. And I seriously doubt you will get many scenario players to hang around on Japanese airfields for a few hours waiting for the B29s from the Marianas, China or Tinian to arrive so they can try and intercept them.
You can at least historically point to the use of the other 4 engine bombers in game to some tactical airwar use supporting the ground war in the ETO or pounding islands in the PTO.
In short, there is no historical justification, or practical use for a B29 in Aces high until the ranges get longer and strategic targets become part of the game.
There are far more planes that would make sense to the tactical airwar game that AH is, long before the B29
Until then the addition of the B29 would just be to grief or to unbalance the gameplay.