Hagman will reprise his role as villainous oil baron J.R. Ewing. Gray will again play J.R.'s wife, Sue Ellen, and Duffy returns as his younger brother, Bobby.
The original "Dallas" ran from 1978 to 1991 as a pioneering prime-time soap on CBS.
The new story focuses on the Ewing offspring as they clash over the future of the family dynasty. Josh Henderson and Jordana Brewster also star.
TNT did not say when the pilot would begin production.