X-planes are what happens when the Air Force gets a huge pile of money to try something nutty. Or, okay, that may not be entirely true, but that big fat "X" stands for "eXperimental," and X-planes do have a noble lineage, from the Bell X-1 to the X-37 robotic space plane. The latest addition to the family is the X-56A, also known as "the MUTT."