The distance it had to travel surely ruled out fighter escort, and however weak Japanese air defences were, a group of fighters would bring down a B-29? Was it just it's altitude and speed?
You can find the answer in the top response at this previous question: https://www.reddit.com/r/AskHistorians/comments/2jigkz/was_there_any_concern_in_the_us_government_that/
In short, at that point in the war the bombers were basically flying unopposed and mechanical failure was a bigger risk than enemy fighters.