There were, through the mid- to late sixties, any number of bands hawking their particular brand of blues and R&B across the British countryside, from the poppier end of the spectrum (The Animals, Yardbirds, Manfred Mann) through to the out and out hard core traditionalists (John Mayall) with outfits like the Graham Bond Organisation somewhere between the two.
There was, if you struck it lucky, the opportunity to rise to something approaching stardom (a la The Rolling Stones) but for most of the practitioners who were doing it for a living it was a pretty hard slog. That changed towards the end of the decade, as other outfits (most notably Led Zeppelin) followed Cream into the American market, but until that happened we’re talking a niche market that was viable but didn’t pay all that well.
Eric Clapton was, as far as such a beast existed, the only significant name on the circuit who wasn’t an actual bandleader (John Mayall, Graham Bond, Zoot Money, for example) and the whole Clapton is God graffiti bit was kicking off when drummer Ginger Baker approached him to sound him out regarding a new band. Baker, one suspects, was looking for a better share of the gig proceeds than the hired hand wages he’d been getting to date.
Clapton, after stints in The Yardbirds, which he’d left because For Your Love was far too poppy for a blues purist to associate with and John Mayall, where he was just about on co-headline status with the nominal leader, was looking for something interesting and had noted the existence of a rather good bass player in the shape of Jack Bruce, who could also sing and also had his eye on the young Steve Winwood in the vocal and keyboards department.
Guitar, bass, drums and Hammond B3 seemed to be the default blues band lineup, aided and augmented by the odd saxophone if the finances stretched that far.
So Baker, looking for a better earner, approaches Clapton, who is coming off seeing Buddy Guy in a trio setting and learns, yes, Eric’s interested, but he’d like to see Jack Bruce on bass. That was a complication Baker wasn’t ready for. He’d worked with Bruce in the Graham Bond Organisation and the pair, regardless of how well they worked as a rhythm section, loathed and detested each other.
There was the odd rehearsal/jam here and there through 1966, basically, one gathers, when everyone was in the same London neighbourhood, but each of the trio had a gig elsewhere until Baker let the cat out of the bag in a newspaper interview. At that point Clapton got the bullet from Mayall’s Bluesbreakers (though he seems to have been ready for a change and looking for a way out) and Bruce was ejected from Manfred Mann, so the trio had little choice but to see whether they could actually make a go of it.