According to musicologist Ian MacDonald, Lennon created the song based on the chords of Del Shannon's "Runaway"[3] a UK hit in April 1961.

Unusually for a pop song, it shifts between major and minor keys, incorporates two distinct bridges, and lacks a traditional chorus.

[5] However, Ian MacDonald observed that "I'll Be Back" took a different aproach: "Fading away in tonal ambiguity at the end of A Hard Day’s Night, it was a surprisingly downbeat farewell and a token of coming maturity".

[3] Music journalist Robert Sandall later reflected on the song's significance in Mojo magazine writing: "I'll Be Back" was the early Beatles at their most prophetic.

This grasp of how to colour arrangements in darker or more muted tones foreshadowed an inner journey they eventually undertook in three albums' time, on Rubber Soul".