Dave Stewart first enlisted Mick Jagger.

Then he wanted Joss Stone to hear what they'd come up with.

And then, rather than turn to the English rock, American blues and R&B at the core of their respective sounds, Stewart hipped Stone to a group she'd never heard of: the Andrews Sisters.

The idea of discovery, of crossing into unfamiliar terrain -- like the 1930s hitmakers -- was the driving concept behind SuperHeavy: What would happen if a band of musicians from different cultures composed and recorded together? Jagger, Stewart, Stone, Damian Marley and A.R. Rahman, five musicians from different backgrounds, experiment with one another's sounds on the group's album, due Sept. 20 on Universal Republic in the United States and A&M in the rest of the world.

Mick Jagger's SuperHeavy Supergroup to Drop Album in September

Marley, son of Bob, says, "For me, it was a great experience to get together and experiment with other musicians. I wasn't familiar with everyone else's music before the project." Rahman, Stewart recalls, "was a bit taken aback when I called." He'd never worked with rock bands or a reggae artist. Neither Jagger nor Stone were ever in vocal groups.

"Normally I do everything -- which I'm happy to do," Jagger says at Jim Henson Studios in Los Angeles where seven of the album's songs were premiered. "It was kind of fun. You pick your part and then get to harmonize. Joss and I would do a harmony together and then Damian comes in with his toasting thing. It was very much a group vocal. I never really worked with a vocal group before so that was a new experience." What they created has no true connection with the Andrews Sisters, but Stone says that listening to their music "created a common ground."

Mick Jagger's New Group SuperHeavy Unveils Music

About a year was spent crafting an album after an initial series of jams and songwriting sessions produced more than 35 hours of music. Stewart and his engineer reviewed the recordings, which occurred in Los Angeles, looking for moments that could be shaped into songs. Jagger says they entered the first session with "ideas, a few guitar riffs and a few snippets of lyrics. Most everyone I have worked with has something [prepared], so it's not my usual sort of way of working. You always want to leave some room for improvisation, but you need to have something, some songs, when you walk into the studio.

Video: Behind the Scenes: Superheavy In The Studio

"It just evolved very quickly, getting the grooves going," Jagger continues. "We sat around with our little pads [writing] . . . but it's all coherent and arranged. We just wrote quickly." Stewart says they recorded 29 songs in 10 days. "Some are an hour and 10 minutes long, some songs are 42 minutes long," he says. "We reconvened and then sort of made it into a shape. In the last year it fell into place."

Next: Behind the Scenes of "Miracle Worker"