In fact, the X-33 does exist, as does the technology to build this type of very high-speed plane, which could be used to transport humans in the near future. 

The “X” that precedes the name of this and many other vehicles of its kind stands for experimental.  This tradition began with the Bell X-1 in 1946, and such X aircraft have continued to push boundaries in both technology and flight concepts.

The original idea behind the X-33 was to reduce the cost of getting a pound of payload into space from $10,000 to $1,000.  Boeing provided the engine, while NASA and Lockheed Martin provided the vehicle. (David)  NASA looked to the project to lead to a space shuttle replacement while Lockheed Martin anticipated a type of “dream machine”, or “a commercial, fully reusable, single-stage-to-orbit vehicle.” (David) 

This is the model Dan Brown envisioned traveling to Switzerland at Mach 15, carrying our hero, Robert Langdon. Unfortunately, NASA has recently decided to withdraw funding from the X-33, and the experimental model has now been dismantled. (NASA)  But this doesn’t mean that such projects have come to an end.  There is already an X-34, and as long as this technology is seen as useful, it will continue to develop.