And what would be the mission of such an aircraft? Is the USAF trying to build a new version of the B-70 Valkyrie?
Uninterceptable cruise missile.
It would be faster than the usual air defense SAMs (like S300 or Patriot) and AAMs.
You would need something approaching the performance of an ABM capable SAM, but those are designed for ballistic missile intercepts. This ramjet cruise missile would be a maneuvering target.