The X-15 research plane was a legendary rocket aircraft when I was a kid in the 1960s: a sleek black beauty that daring silver-suited pilots steered to the edge of space.
It turns out that its accomplishments fifty years ago have become useful once again in the current development of the state-of-the-art spacecraft SpaceShipTwo and DreamChaser. BBC has published an article on these aspects of the X-15 project – link >>>
And here is a period NASA documentary on the original X-15 project — link >>>