What episode did Naruto become chunin?
Naruto Shippuden: Season 18: Episode 22
Does Naruto ever officially become a chunin?
No Naruto never becomes chunin in the series nor is he ever chunin. After the war and Kakashi becomes hokage, Naruto studies to become straight to jonin (anime) bypassing the chunin stage. It obviously makes sense that Naruto never becomes chunin as he is well over jonin level by the end.