Got this from "Paperback Writer Beatle Music History":
It was indeed Paul playing lead guitar which leaves the bass guitar work to George Harrison, since this rhythm track did not have any overdubs at this time. Adding to this conclusion are photos of the session on this day which shows George playing another new instrument to The Beatles line-up, a Burns Nu-Sonic bass guitar. John’s Gretsch electric guitar was apparently never used again by the group after this day. George, however, appears to have used this short-scale Burns bass guitar a couple more times during the “Revolver” sessions.
Probably George just wanted to try out something new and picked that instrument.