During the second supersonic flight of the XB-1 technology demonstrator today, the jet exceeded Mach 1 three times without a boom being detected by microphones on the desert floor below.

Boomless Cruise is based on physics known as the Mach cutoff, in which a sonic boom refracts in the atmosphere and never reaches the ground. This was achieved by breaking the sound barrier at a higher altitude, with the exact speeds varying based on the atmospheric conditions.