The APU on a B-29 must stay running while taxiing in order for the batteries to have enough "juice" to keep the electric/hydraulic pump pressured up for the brakes. The airplane has no direct nosewheel steering, as the nosewheel just freely casters. Steering is done solely by the brakes and alternate engine power. When the engines are at idle, their generators aren't turning fast enough for them to be "on line," so that is why the APU stays running.
As a matter of fact, the APU stays running until AFTER the gear is raised after takeoff, since it requires 460 amps to raise the gear.
There are seven generators on a B-29. Two on each outboard engine and one on each inboard. Those are 300 amps each. The APU generator is 200 amps. If any of the engine driven generators aren't working properly, the APU will assist the gear and flap movements (the flaps require 270 amps to operate), along with any other electrical draw that is required of the airplane.
(NOTE: the following statement was taken from one of the crew members of the Kee Bird Recovery Team. Not all of them tell the story the same way, so there may be some different variants to this story. I personally tend to believe this one though...) "The fuel pump on the Kee Bird's APU had failed, so it couldn't draw the fuel from the factory installed fuel tank anymore. A 5 gallon gas can was hung from the top fuselage formers with a coat hangar, so that the fuel could be gravity fed to the APU. This worked well until the can broke loose from the coat hangar while taxiing, spilling the fuel on top of the hot APU. The ensuing fire consumed the airplane and could not be extiguished."
I'm sorry if this post is too technical, but I frequently hear how "the APU burned that B-29 to the ground," and it drives me nuts. A simple mistake (and getting in a rush) is what caused the airplane to burn to the ground. Those guys did what they could with what they had, after all, they were out in the middle of a frozen lake in the Artic. However, it just shows how something so simple going bad can be so tragic.
Oh, and by the way, I recently installed two more batteries to the electrical system in FIFI, which now gives the airplane three batteries in the system. The idea of this was to eliminate the need for the APU, which was confirmed in our series of test flights before the engines on the airplane went ka-put. We do, however, keep the APU on the "ready" in case it's needed for an electrical emergency.
Gary