This week's AW&ST has a "future of flight" theme and one of the TOC items is "Boeing Unveils XB-70-Inspired Hypersonic Launcher Concept". As a non-subscriber I won't know more until I get myself to a library, but it sounds like it's propelled by eight "Mach 4 turbines" and some kind of RBCC system for the final kick.
Just to prove that the only thing new in the world is the history you don't know, NTRS just posted a 1959 Boeing study of basically the same idea as a reusable first stage for the X-20, with an expendable second stage.
Just to prove that the only thing new in the world is the history you don't know, NTRS just posted a 1959 Boeing study of basically the same idea as a reusable first stage for the X-20, with an expendable second stage.