According to a press-release from BBC America:
David Bradley (Harry Potter) is set to play the first ever Doctor, the iconic William Hartnell, in the drama written by Mark Gatiss (Sherlock, Doctor Who), who will also serve as executive producer alongside Steven Moffat (Doctor Who, Sherlock) and Caroline Skinner (Doctor Who, The Fades). An Adventure in Space and Time (1x90) is a co-production between BBC Cymru Wales and BBC AMERICA. The drama will premiere later this year on BBC AMERICA."I’m absolutely thrilled," said Bradley. "I first heard about this role from Mark while watching the Diamond Jubilee flotilla from the roof of the National Theatre. When he asked if I would be interested, I almost bit his hand off! Mark has written such a wonderful script not only about the birth of a cultural phenomenon, but a moment in television’s history. William Hartnell was one of the finest character actors of our time and as a fan I want to make sure that I do him justice. I’m so looking forward to getting started."
The show’s writer and executive producer, Mark Gatiss said he's "utterly delighted that everyone's favorite Time Lord will be in such brilliant and stellar company."
"We have a terrific team who can't wait to tell the fascinating and surprising story of how The Doctor began his journey through Space and Time."
Filming of the special will begin in February.
The second half of Season 7 of Doctor Who, premieres on BBC America March 30.