Board Thread:Naruto Discussions/@comment-30168130-20161120204250/@comment-4912646-20161121064420

For what it's worth, the movie guidebook that accompanied The Last confirms that the black markings that spread across Naruto's body in chakra mode are his Four Symbols Seals made visible, and Minato seemingly has the same seal on his chest. The mode also debuted in the chapter titled A New Seal!! and there was considerable emphasis placed on Naruto's seal in the events leading up to it, not to mention the reveal that it was created by the Uzumaki clan also came around that time.

So all things considered, the unique properties of Naruto's seal seem like the most likely explanation for why he has a special form and B doesn't, though it sucks we never got any official confirmation.