Batman: Arkham Knight was originally due out this year before being pushed back to 2015 in order to deliver an "awesome level of quality."
While Kevin Conroy, the voice of Batman in the game, told Groovey.TV last week that Arkham Knight would be released in January, a new listing on the Microsoft store indicates that the game could instead be released on February 24, 2015. Warner Bros. has yet to officially announce a date for the game, so I'd treat this as rumor and speculation for now.
We had a chance to see Batman: Arkham Knight at E3 earlier this month, with writer Lance Liebl going hands on with the new Batmobile. He claims "it's the best thing to happen to Batman since Christopher Nolan." Bold words.