The "P" factor is simply the torque of the engine causing the plane to "yaw" to the left. This can be a serious issue with twins if there is no off set of the engines to compensate for it and if you have an engine out. The arf probably has it already built in. I'm sure it will also cause a 4 engine plane to yaw a bit as well. But with the very large vertical stab and long wing span of the B-29, it might not be as critical. I will definitely put a few degrees of right thrust on the engines.

If anyone out there is still flying one, feel free to comment on how the plane acts and if you have any right/ down thrust compensation with the engines and how much.