Now according to Wikipedia they have the same climb rate but is that empty or heavy?
The B-29's cruising speed is around 220-225mph and maximum speed is around 350mph compared to 182mph and 287mph for the B-17. Even though both have approximately the same climb rate, the B-29 will get you to the target faster, with more bombs and with superior defensive armament, probably to the target intact.
Again, thinking that the B-29 will not render any of the other piston engine bombers hanger queens is a little naive. Why up something else if the B-29 can get you there faster, with more bombs and a better chance of survival? If someone decides to take the B-29 up to altitude, it will be virtually untouchable.
ack-ack