On B-29's, as far as I am aware there are only two airworthy B-29's left; "Fifi" and "Doc". They recently flew together at the EAA AirVenture in Oshkosh Wisconsin. If you're a fan of round engine noises, these are some of the largest ones left.
https://www.youtube.com/watch?v=ncUdqT8AxY0