A handful of aviation companies are working to bring back supersonic passenger travel and one of the newer contenders, Boom, recently completed the first round of wind-tunnel testing, a critical step in the development of a prototype. The company’s next step is to start assembly of the XB-1 Demonstrator, a technologically representative one-third-scale version of the production Boom airliner, which will ultimately seat up to 45. The first flight of the XB-1 is expected late this year.