Boeing's X-20 Dyna-Soar Space Plane Could Have Hit Mach 18
The Air Force and Boeing developed the X-20 Dyna-Soar (Dynamic Soaring) from 1959 to 1962. Intended as a reusable recon and research craft boosted by a Titan rocket, the craft was also envisioned as a hypersonic bomber. Indeed, the X-20 had all the makings of a game-changing platform before it was cancelled in 1963.
Source: nationalinterest.org