Progiest non-prog album

The Steve Miller Band's 1968 album "Children of the Future" was recorded at Olympic Studios in London under the direction of legendary English producer Glyn Johns. With a hint of Summer of Love psychedelia mixed with West Coast sound, Side One was mysteriously veiled in the hazy artistry of the British progressive rock ethic. 

The first track is the title tune. It's bluesy, exploratory, and has elements of folk-rock, yet it also fits in nicely with the progressive music era. The title track's keys and swirls give way to "Pushed Me Through It" and the mellotron-driven prog-rock song "In My First Mind," which flow into the epic, hazy, lazy, organ-infused "The Beauty of Time Is That It's Snowing," which fluctuates in unexpected ways. Side One of the "Children of the Future" LP, which is listened to as one suite, is one of the best works that the Steve Miller Band has ever done and is one of the best examples of early progressive rock.

The music on Side Two is more in line with "normal" blues-rock music. However, it is just as delicious as side one. To kick things off, we have my all-time favourite Boz Scaggs song, "Baby's Callin' Me Home," with its majestic acoustic guitar and harpsichords. "Steppin' Stone" features strong rock music from the 1960s, while "Roll With It" is another rock song about life on the road, including sharp guitars, a swaying organ, and Steve singing from a very high place. "Junior Saw It Happen" is the most vibrant of the final three songs. Ben Sidran's harpsichord performance on "Baby’s Callin’ Me Home" is peculiar, but Miller's mouth harp on "Key to the Highway" is fairly awesome.

One of the first bands ever to present their music as a seamless flow of sounds rather than a collection of individual pieces was The Electric Flag on their first studio record. This sounds proggy, doesn't it?

Well, released in April 1967, "The Trip: Original Motion Picture Soundtrack" is, due to its structure and the year it was released, certainly a progressive music effort, yet in an American way. Consisting of 18 tracks, the album features an eclectic variety of musical styles, from epic scores ("Peter's Trip") to acoustic wanderings ("Joint Passing"), odd circus-like music ("The Other Ed Norton"), pliable floating substance exhibiting hallucinogenic effects ("M-23"), wild saxophone solos set to furious jazz rhythms ("Peter Gets Off"), electric-violin mood swings ("Synesthesia"), fluid fringe (as in "Flash, Bam, Pow "), trippy states that limp ("A Little Head"), reeling welters ("Fewghh"), a fierce improvisation of blues-rock ("Gettin' Hard), but once the mists lift, we're treated to breezy horn -based tunes ("Green and Gold"), and a wild seven-minute jam called "Fine Jung Thing" as well.

Moreover, "The Trip" is among the first rock records to use the Moog synthesiser, brought by electronic music pioneer Paul Beaver.

Boston- Boston

Supertramp- Even in the Quietest Moments

I don't see how Supertramp aren't prog. They are even on this site as crossover prog. That album even has the very much prog "fool's overture." There's even a case for Styx who are also on here but as prog related. Boston maybe not so much but even that first album has it's moments. Maybe semi prog should be a thing but again you'd have to have a pretty snobby attitidue about what is and what isn't prog to not consider Supertramp to be some kind of prog imo.

The Coop's proggy nature can be seen much earlier. I already mentioned the album Killer, but you can hear the band's prog-leanings on several songs from both Love It to Death and School's Out. I'd mention Easy Action, but I think their longer songs on that album were more acid rock/psychedelia.