Although it wasn't a prog album (very post-modernist self-satirizing pop, I'd define it as), the first concept album ever is contested by some to be "Freak Out!" by Frank Zappa and the Mothers of Invention. According to several sources I've read, Paul McCartney apparently once said it was this album that inspired the Sgt. Pepper concept.
MY personal thought on the matter is that calling ALL of "Freak Out!" a concept is stretching it a bit, but it had some concept elements (Suzy Creamcheese), some definate experimentation ("The Return of the son of Monster Magnet"), and was the longest album of its time (the first 2-LP rock album ever). I think the Mothers' next album, "Absolutely Free" was far more undeniably a concept album, and I'm pretty sure it was recorded before Sgt. Pepper as well.
That being said, this takes nothing away from the Beatles, though. They influenced ALL music that came after them. I've used the analogy that the history of popular music via it's influences is an hourglass, and the Beatles ARE the center of that hourglass. EVERYBODY, whether they like it or not, was/is influenced by the Beatles.