Without knowing what is being altered I assume the B-29 may not get past planning or appropriations in many alternate timelines. The B-29 was part of the desire to reach Europe, if that goes away or more muted in danger then the B-17 and its ilk likely remain "good enough" to the purse holders. But I agree, even as is the B-29 was accelerated to fit the war we had rather than where we planned to be. I will leave the Manhattan Project on the sidelines, it too is subject to butterflies. But the later needs a B-29, or better the B-36, to be where the Air Force sees itself going. But will Congress pay for that without a very direct threat?
My interest is seeing a USA at war with what it had not with what it could have. The USA was a peer naval power but a lesser military power otherwise, beyond its core the Army was more police force than anything else, the Marines did well in the Banana Wars or policing in China, but the USA pre-1940 is more aspirations than substance. High potential indeed but it needed an enemy to motivate that, take out Europe and the Army Air Corps is scouting in B-17s hoping to bomb something lest the Navy do it better. I think we need to know better the backdrop to this, otherwise I am open to the B-29 never getting more than sketch art.