Atlus announced Etrian Odyssey V back in November 2014 without a platform. The studio said in January that the game is in its final stages of development, but Atlus still hasn’t announced its platform.
A new Atlus survey that went live today might have confirmed the game’s platform. In one of the questions, it lists Etrian Odyssey V as a 3DS title:
A release window for Etrian Odyssey V is still to be announced, but we should start getting information when the weather warms up, according to Atlus.