According to Air Force statistics XX Bomber Command lost 80 B-29's between Jun-44 and Aug-45
22 were lost due to enemy fighters
7 were lost due to enemy anti-aircraft fire
51 were lost due to mechanical, operational, or other accidents.
Also, XX bomber Command reported the following damage incidents due to all causes
419 major damage incidents between Jun-44 and Aug-45
2,267 minor damage incidents between Jun-44 and Aug-45
It looks like the B-29 was pretty durable, but then again, statistics can be misleading.