Reviewed by Ian MacDonald, in Revolution in the Head:

This fascinating Lennon song, probably written during his holiday in May, occupied the evening session of 1st June. A personal favourite of his, it was based on the descending chords of Del Shannon's 1961 No. 1 'Runaway', though loosely enough to be unnoticeable except to other guitarists. Starting, as it ends, with an irresolute swing from A major to A minor (a triplet plus a dislocating push which recurs in both the bridge and the middle section), I'LL BE BACK, is a melancholy essay in major/minor uncertainty mirrored in the emotional instability of its lyric. The most unorthodox thing Lennon had yet written, it has no real chorus, being constructed as a twelve-bar verse in two equal sections (shortened to six bars in its final reprise), a six-and-a-half bar bridge, and a nine-and-a-half bar middle featuring a characteristic chromatic descent [1]. Despite all this, and as so often with Lennon's more bewildering constructions, the whole rings totally true, being his deepest emotional expression to date.

With its warmly resonant acoustic backing track, I'LL BE BACK is driven by a swinging beat loosened by a cross-rhythm of flamenco-style half-time triplets [2]. Lennon is harmonised by McCartney in shifting major and minor thirds, resolving on a Picardy third at the end of the first and second verses. (Someone - McCartney or Harrison - holds the upper E of the harmony throughout.) The singing could have been subtler and the harmony is at points sufficiently recondite that McCartney's pitching wobbles, but as a whole this is one of the most concise and integrated songs The Beatles had so far created. 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.

Notes:

1. Some describe the song as having 'two different bridges' or 'two different middle eights'. (On Anthology 1, take 3 is faded on a repeat of the last two bars of the second bridge.
2. An experiment in 6/8 was tried on take 2, but got no further than the bridges before Lennon broke down, unable to sing it. In his notes to Anthology 1, Mark Lewisohn takes this to indicate that Lennon conceived the song as a waltz, claiming that take 3, with the group back in 4/4, offers an example of how quickly they could alter arrangements. However, I'LL BE BACK was clearly composed in common time, the anomaly being the experimental 6/8 version (perhaps suggested by McCartney or Martin).