Here's what I think of the B-29- it is the very best of the breed from any country. It completely dominates the field from nearly every perspective. If the AH staple of adding several thousand feet of performance to bombers holds true, you would see routine flights of 360mph 35K buffs. Placed in groups of four, you would then have formidable opposition that unperked fighters couldn't catch, and perked fighters wouldn't dare approach.
And really, if you aren't going to perk the best of the breed, what are you going to perk?
On a game balance issue, what would the Axis have to counter? While this is immaterial to MA play to a point, there is the valid point of trying to fairly represent all sides. Give the Axis the He-177 first, then talk B-29.
Still, for my money I would rather see far more useful medium bombers. They are typically fast enough to get in and out and carry sufficient bombloads to get the job done. Witness the Ju88 or B26- both carry bombloads approaching the B17, yet both perform better (aside from high alt). Granted they haven't the defensive fire, but they have much better speed. And the tactical nature of the MA makes using mediums easier and more efficient.