| Manufacturer | Designation | Name | Note(s) |
|---|---|---|---|
| Bell | X-1 | Formerly XS-1 | |
| Bell | X-2 | Formerly XS-2 | |
| Douglas | X-3 | Stiletto | Formerly XS-3 |
| Northrop | X-4 | Bantam | Formerly XS-4 |
| Bell | X-5 | Formerly XS-5 | |
| Convair | X-6 | Projected nuclear-powered research variant of the B-36 | |
| Lockheed | X-7 | ||
| Aerojet General | X-8 | Aerobee | |
| Bell | X-9 | Shrike | |
| North American | X-10 | ||
| Convair | X-11 | ||
| Convair | X-12 | ||
| Ryan | X-13 | Vertijet | |
| Bell | X-14 | ||
| North American | X-15 | ||
| Bell | X-16 | ||
| Lockheed | X-17 | ||
| Hiller | X-18 | ||
| Curtiss-Wright | X-19 | ||
| Boeing | X-20 | Dyna-Soar | |
| Northrop | X-21 | ||
| Bell | X-22 | Model D2127 | |
| Martin Marietta | X-23 | PRIME | |
| Martin Marietta | X-24 | ||
| Bensen | X-25 | ||
| Schweitzer/Lockheed | X-26 | ||
| Lockheed | X-27 | Lancer | |
| Pereira | X-28 | Osprey I | |
| Grumman | X-29 | ||
| Rockwell | X-30 | NASP | |
| Rockwell/MBB | X-31 | EFM | |
| Boeing | X-32 | JSF | |
| Lockheed Martin | X-33 | ||
| Orbital Sciences | X-34 | ||
| Lockheed Martin | X-35 | JSF | |
| Boeing | X-36 | ||
| Boeing | X-37 | Future - X | |
| Scaled Composites | X-38 | Lifeboat | |
| X-39 | |||
| Boeing | X-40 | SMV | |
| X-41 | |||
| X-42 | |||
| MicroCraft | X-43 | Hyper-X | |
| Lockheed Martin/Boeing | X-44 | MANTA | |
| Boeing | X-45 | UCAV |