Some climb to 30k with the 17's, 24's and Lanc's.
You're incorrect on the speeds.
B-17:
Maximum speed: 287 mph
Cruise speed: 182 mph
Short range missions (<400 mi): 8,000 lb (3,600 kg)
Long range missions (>800 mi): 4,500 lb (2,000 kg)
B-29:
Maximum speed: 357 mph (310 knots, 574 km/h)
Cruise speed: 220 mph (190 knots, 350 km/h)
Bombs: 20,000 lb (9,000 kg) standard loadout
Ki-67:
Maximum speed: 334 mph (537 km/h)
Boston III:
Maximum Speed: 344 mph (550 km/h) at altitude.
Bombs - 4,000 lb (1,900 kg)
In short, the B-29 would be next to impossible to catch with a piston engined airplane, given the space and area within the game. If you're underneath it, forget it. To "catch up" would take more sectors than it already does. If it were to be added, the payload could not be 20,000 lbs. You could potentially take out 3 fields in a single sortie with just ONE B-29.