I'm no Beatles expert, but I did just read the Geoff Emerick book. He goes into quite a bit of detail about recording Paul's bass, and I don't recall any mention of John or George playing bass at all. On the other hand, apparently Paul played a lot of the guitar parts that George couldn't pull off!
By "George couldn't pull off", you mean "George couldn't reproduce exactly as Paul wanted." Watch the Let It Be movie for an example. I love Paul, but the fact that he told George and John what to play with increasing frequency was a big part of the Beatles breaking up.