An 1985 Article about Miller's Dark Knight...BEFORE the book was released.

Dork Note: I always thought Batman in Dark Knight was in his late fifties or early sixties...yet here it says he is 49! Holy shit, Batman...that's not that old!

Clipped from Wikipedia: In the early 1980s, DC Comics promoted Dick Giordano from his position as Batman group editor to editorial director for the company. In Giordano's words, his chief contribution in this position was "getting people to work for DC who could do the work a little better". This resulted in the recruitment of writer-artist Frank Miller to create The Dark Knight Returns. Giordano said he worked on the story's plot with Miller, he commented, "The version that was finally done was about his fourth or fifth draft. The basic storyline was the same but there were a lot of detours along the way."

Giordano dropped out of the project halfway through due to disagreements over production deadlines. Giordano said, "Frank wanted to take the time that was needed to get the job done". Comics historian Les Daniels comments that Miller's idea of ignoring deadlines was "the culmination of the quest towards artistic independence". DC published the issues of The Dark Knight Returns in packaging that included extra pages, square binding, and glossy paper to highlight the watercolor coloring by colorist Lynn Varley.

Miller has claimed in interviews that inspiration for the plot came from Dirty Harry, perhaps specifically the 1983 film Sudden Impact, which like DKR focuses on an older Dirty Harry returning to fight crime after a lengthy convalescence. Miller also cites his increasing age as a factor in the plot.