I've found this:
PAUL circa-1994: "'Flying' was an instrumental that we needed for (the film) 'Magical Mystery Tour' so in the studio one night I suggested to the guys that we made something up. I said, 'We can keep it very, very simple, we can make it a 12-bar blues. We need a little bit of a theme and a little bit of a backing.' I wrote the melody, otherwise it's just a 12-bar backing thing. It's played on the mellotron, on a trombone setting. It's credited to all four (Beatles), which is how you would credit a non-song."http://www.beatlesinterviews.org/index2.html
In that page you'll find Beatles quotes about almost every official song they did.