They were testing (at a different scale, with a different powerplant) their air intakes for the engine which are designed to enable conventional turbofan engines to operate at supersonic speed, plus some control systems. And no doubt demonstrating to investors that they could fly something at supersonic speed before they ask for the funds to design and build a new powerplant and airliner-scale airframe...
It was originally envisaged as a maiden flight that would happen within a couple of years of founding, but aerospace is hard.
When taking another look at the Wikipedia articles for the XB-1 and the Overture, I also noticed that both of them mention the fact that the XB-1 still uses the original trijet engine configuration planned for the Overture, which has however since been changed to a quad-jet (https://airinsight.com/boom-supersonic-radically-changes-ove...). So the XB-1 is even less representative of the full-scale Overture than I thought...
they're early enough in their program to switch the Overture again to twinjets :)
(only half joking, if it turns out that adjacent engines in the quadjet configuration have a negative impact on their intake technology, a twinjet would work more similarly to the trijet...)
It was originally envisaged as a maiden flight that would happen within a couple of years of founding, but aerospace is hard.