Back in February, we reported that the Doobie Brothers were going to re-record their biggest hits with some of country music's biggest stars. We got a taste of what that might sound like last night (June 27), as the Doobies joined the Zac Brown Band at Boston's Fenway Park.

Towards the end of their main set, several members of the group, including Tom Johnston, Patrick Simmons and John McFee, came out to sing 'Black Water.' So even though the track listing for the duets album has not yet been announced, their performance gives a fairly strong indication that it will appear. In the encore, they also performed a cover of Pink Floyd's 'Comfortably Numb.' It was the second consecutive night that Brown made an appearance at Fenway. On Thursday, he sat in with Billy Joel on stage to sing 'You May Be Right.'

In addition to the Zac Brown Band, Brad Paisley, Sara Evans and Toby Keith perform on the album, which is expected to be released in the fall. Five-time Grammy winner Michael McDonald, who has returned to the fold for this recording, told Billboard that Evans sings on 'What a Fool Believes' and that Love and Theft perform 'Takin' It to the Streets.' Johnston recently told New Hampshire's Union Leader that most of the music was performed by session musicians, with the person who wrote the song contributing on their instrument.

"For instance, if it was 'China Grove,' he said. "I was playing guitar on it; if it was 'Black Water,' Pat was playing on it; if it was 'Takin’ It to the Streets,' it was Michael playing keyboards on it. But other than that, it’s all Nashville guys doing all the playing, and they were incredible. So, [we] got to have instrumentation like mandolin or banjo and pedal steel. That puts a whole different color on the tune. The driving playing behind it was just killer, and then whoever sang it, of course, had their own interpretation ... and that put another color on the song. It was just really cool."