What design features do XB-1 and Overture incorporate to enable boomless operation?

“Unlike "low boom" designs, Boomless Cruise does not require extensive aircraft shaping. It relies on breaking the sound barrier at high altitude where the boom refracts away. Symphony engines enable efficient supersonic transition above 30,000 feet. An advanced autopilot uses weather conditions and software algorithms to automatically select optimal Boomless Cruise speed.”