George Harrison was a Beatle and then he went solo. I would even posit further that the Travelling Willburies was George Harrisons first band were he was a true equal to his bandmates.I'll clear up one thing you might not know. George Harrison was in a band called The Beatles, then he went solo, then Willburys. But he was not known as a solo artist...he was a Beatle.
Dylan and Orbison may fit the question. Successful solo artists who joined a band. But Petty? Is Tom Petty and the Heartbreakers really a solo project?
I would classify Tom Petty as a solo artist the same way Bruce Springsteen is. Both are the band leaders and both hire their backing band. The E Street Band did not sell their catalogue as they do not have one as they are employees/independent contractors of Bruce Springsteem Inc. (I do not know what his corpration is called). Similarily, the Heartbreakers was the name of Tom Petty's backing band. The Heartbreakers only existed to support Tom Petty and were employees/independent contractors of Tom Petty Inc.