Sneaky War Dept guys looking to extend procurement of the cancelled B-29 by calling the upgraded B-29 a new aircraft.
Or so says Wikipedia and I dont trust them very much.
That's about right. The B-29 was rushed into production and had a lot of bugs that what became the B-50 was designed to fix. Passing it off as a 'new' design was fairly harmless, as we needed something to bridge the gap until the new jet bombers came on line and actually building a new design would have been a waste of time.