Donald Trump gave an interview to a local Ohio television station on Wednesday during which he was asked why he suddenly changed his mind on his birther lie last week, after pushing it for many years. “Well, I just want to get on with, you now, we want to get on with the campaign,” Trump said to ABC6 in Ohio.
http://www.thedailybeast.com/cheats/2016/09/21/trump-made-birther-comment-to-move-on.html?via=desktop&source=copyurl
That tells me he either still believes that Obama wasn't born in the US or has always believed that Obama was born in the US, but just did the whole birther thing for publicity.