B-29 Wing Design

[Lippert]

The B-29 wing is still tapered, it just sports a more tapered leading adge than trailing edge. A swept wing would have a swept leading and trailing edge.

In my opinion, I think it has a lot to do with the size of the wing itself. Its spars at the time were the largest aluminum extrusions made. To give the aircraft great range, it was a design requirement that it have a very high aspect ratio wing, and the only way to do this with the materials avalable was to create a wing that was long and tapered - as you have mentioned with the other bombers. The lessons learned from those bombers, however led to better understanding of the need for very high aspect ratio wings, which the B-29 has a much higher one than its predeccesors.