The project, which comes from Glee producer Ryan Murphy, focuses on a gay couple and the woman they ask to be their surrogate.
Rannells has now signed up to play one of the leads in the comedy, Deadline reports.
Rannells has appeared in a number of Broadway productions but is possibly best known for starring alongside Josh Gad in The Book of Mormon. He is also due to guest star in HBO's new show Girls.
NBC has not officially ordered Murphy's project to pilot but it is widely expected to do so shortly.
Allison Adler, whose previous credits include Just Shoot Me!, Chuck, Glee and No Ordinary Family, is also working behind the scenes on the show.
Other projects currently in the works at NBC include a comedy starring Marcia Gay Harden, a soap opera adaptation of Dracula, a sitcom from Seinfeld writer Spike Feresten and sci-fi drama Beautiful People, starring Frances Conroy.