The sci-fi series marks its 50th anniversary this year (13) and BBC bosses have commissioned a special feature about the drama as part of the celebrations.
The TV movie, titled An Adventure in Space and Time, tells the story of the show's creation, and Bradley, best known for playing school caretaker Argus Filch in the Potter films, will play Hartnell - the first actor to tackle the title role.
Manhunter star Brian Cox will also appear in the film as the BBC's former head of drama Sydney Newman, and the script has been put together by Mark Gatiss, a regular writer on Doctor Who.
Bradley says of the project, ""I first heard about this role from Mark (Gatiss)... When he asked if I would 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.""
Gatiss adds, ""What a cast! I'm utterly delighted that everyone's favourite 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.""
An Adventure in Space and Time will begin filming next month (Feb13) and will air in the U.K. later this year (13).