Though the X-84 Starfighter is the first Single Stage To Orbit (SSTO) craft to make it into military use, it is not the first to ever consider the challenge. In fact, the idea of a totally reusable ship has been around since the advent of the Space Shuttle, if not before.
The phrase "Single Stage To Orbit" means simply what it says: unlike the Saturn V rockets that lifted the Apollo missions into space and unlike the throw-away fuel tank and reusable booster rockets on the Space Shuttle, the SSTO craft would be one piece that could launch and return to earth, only to be reused in the future.
If not the first, certainly the most well-known of the earliest working prototypes of an SSTO ship was the Delta Clipper Experimental (DC-X) and its successor the DC-XA. It could also be correctly described as the ancestor of the X-84 we have today. Designed and built by McDonnell-Douglas from 1991-1993 and first flown on August 8, 1993, the DC-X was a cone-shaped spacecraft that could take off and land vertically. It was intended to be a 1/3 scale version of a proposed single-stage launch vehicle. Though it never actually entered space, it performed quite well in atmospheric tests. Eventually, the DC-X was modified into the DC-XA (Delta Clipper Experimental, Advanced). Due to an accident and lack of funding, this project was scrapped after July 31, 9996.
Even before the DC-XA's accident, NASA chose Lockheed-Martin's design for an SSTO craft and commissioned their "Skunk Works" to work on the prototype. Their prototype, the X-33, was a 1/2 scale version of the final VentureStar design they wished to construct. Though the prototype's construction was plagued with delays and equipment failures, once it was finally operational, things looked somewhat promising. However, the full-production VentureStar was never produced. Due to spending cuts and governmental "red tape", the project was moth-balled in favor of completing the International Space Station and working on the missions to Mars. The SSTO project would have to be put on hold for several years.
However, the big name companies weren't the only ones involved in attempting to create a reusable launch vehicle. Many civilians wanted to build a ship that could carry them into space and return, for a fraction of what the big companies would spend. This sparked the X-Prize Competition, which basically would have rewarded $10,000,000 to the first person or group of people to design, build, and fly a reusable spaceship to the edge of space and back. However, after several fatalities and near-fatalities, the X-prize was discontinued to protect the public.
While the SSTO idea took a back seat to other missions, McDonnell-Douglas (the designer of the DC-X and DC-XA) began work on another, single-occupancy ship. This time, they called in Lockheed-Martin to help them on the project. They felt that only a joint effort could produce the vehicle they were looking for.
Designers at McDonnell-Douglas were intent on reviving their DC-X vertical take off and landing concept, with the knowledge that came from their experiences with it. And Lockheed-Martin brought in their experience with the X-33 and the never-produced VentureStar. When NASA OKed production, some of the companies that worked on the X-33 were brought in to help, including Boeing's Rocketdyne Division (the designer of the X-33's linear aerospike engine, BF-Goodrich Aerospace (who designed the heat-resistant tiles of the X-33), and Allied Signal (who helped with the avionics and other electrical systems).
Together, they produced what was to be named the X-84 Starfighter. This ship could only carry a single passenger (except for the trainer version, which held two) and it was designed to be a military craft instead of a scientific ship. After extensive testing, the design went into production. After the prototype, which Joshua Carpenter still flies, only four others were produced.
When the United States military dropped the project in 2028, the members of the team resurrected it as a non-military operation, while keeping the base at the secret test site in Nevada. After some setbacks, the ships were unveiled to the public on July 4, 2029, and they have been serving the good of the world ever since.