The collaboration between DuBose Heyward and George and Ira Gershwin was very close, and complex.
Dubose Heyward wrote the novel, "Porgy," published in 1925. The text is online:
http://xroads.virginia.edu/~HYPER/PORGY/porghome.html
Porgy

In 1934, the Gershwins and Heyward got together at Folly Beach, near Charleston, where they could observe the Gullahs on St. James Island who became the models for Catfish Row residents.

DuBose Heyward wrote the libretto, and George and Ira Gershwin wrote the lyrics. Heyward, however, composed the lyrics for "Summertime" and "My Man's Gone Now."
George and Ira left in August, and George finished the recitatives and orchestrating the opera- some 700 pages of music.

Todd Duncan was selected to play the part of Porgy, after first being rejected because he was a professor at Howard University as well as being a singer. George G. didn't like the idea of a professor taking the part. Anne Brown was selected to play Bess.
"Porgy and Bess" opened in New York in 1935 and ran for only 124 performances. Some years passed before the show's backers got their money back.
http://www.classical.net/music/comp.lst/works/gershwin/porgy&bess.html
This is a very interesting write-up.
Porgy and Bess