Inspiration
On 9 October 1968, while John Lennon and George Harrison were working on two other songs for the album, McCartney recorded five takes of the song in Studio One at Abbey Road Studios. Unlike its heavy blues result,

Recording
Lennon was unhappy that McCartney recorded the song without him. In his 1980 interview with Playboy, he said:
Playboy: "Why Don't We Do It In The Road?"
Lennon: That's Paul. He even recorded it by himself in another room. That's how it was getting in those days. We came in and he'd made the whole record. Him drumming. Him playing the piano. Him singing. But he couldn't—he couldn't—maybe he couldn't make the break from the Beatles. I don't know what it was, you know. I enjoyed the track. Still, I can't speak for George, but I was always hurt when Paul would knock something off without involving us. But that's just the way it was then.
Playboy: You never just knocked off a track by yourself?
Lennon: No.
Playboy: "Julia"?
Lennon: That was mine.

Credits

Lowell Fulson covered the song on his 1970 album In A Heavy Bag.