The War Doctor is an incarnation of the Doctor, the protagonist of the BBC science fiction television programme Doctor Who. He was portrayed by the English actor John Hurt. Although he precedes Christopher Eccleston's Ninth Doctor in the show's fictional chronology, his first onscreen appearance came eight years after Eccleston's; the War Doctor was retroactively created by showrunner Steven Moffat for productions celebrating the show's 50th anniversary.
Within the programme's narrative, the Doctor is a centuries-old alien, a Time Lord from the planet Gallifrey, who travels in time and space in his TARDIS, frequently with companions. When the Doctor is critically injured, he can regenerate his body, but in doing so, gains a new physical appearance and with it, a distinct new personality. This plot device has allowed a number of actors to portray different incarnations of the Doctor over the show's long run.
The War Doctor, not so named within the episodes in which he appears, is introduced as the incarnation of the Doctor who fought in the Time War of the show's modern-day backstory. He was created as a result of a conscious decision of the Eighth Doctor, played by Paul McGann, to take up arms and become a warrior; in accepting this duty, the War Doctor disowned the title of "Doctor", and after the war's end is viewed with disdain by his subsequent incarnations, who reclaim the title that the character is known by. In the 50th anniversary special "The Day of the Doctor", however, the Eleventh Doctor played by Matt Smith revises his opinion of this incarnation after revisiting the final moments of the war.
In his original conception of the show's anniversary special, Moffat had written the Ninth Doctor as having ended the Time War. However, he was "pretty certain" that Christopher Eccleston would decline to return to the role, which he did. As he also had reservations about making Paul McGann's Eighth Doctor the incarnation who had ended the war, he created a never-before-seen past incarnation of the Doctor, which allowed him "a freer hand" in writing the story, acknowledging that the success of doing this would be predicated on being able to cast an actor with a significant enough profile.
John Hurt -- Who Not Who!