During the 1950's when designers created their proposals for what would become the X-15 the Douglas D684 stood out as being lighter, faster & cheaper than the others.
It was also made of a slightly radioactive alloy.
Ultimately it lost the competition for this decision, but not for the reason you might think.
If you want to read more I suggest the book 'X-15 - Extending The Frontier Of Flight' by Dennis R. Jenkins - available for free on NASA's website.
https://www.nasa.gov/connect/ebooks/a...