The list of airlines that want to bring back supersonic travel is growing. Boom Aerospace, an ambitious aerospace start-up, says it has convinced five airlines to buy into its vision for developing a airliner that flies faster than the speed of sound. Blake Scholl, founder and chief executive of Denver-based Boom, said during a presentation at the 2017 Paris Air Show that the five carriers have reserved their spots for 76 of Boom’s successor to the Concorde. That aircraft was taken out of service in 2003 after 27 years of commercial flights at more than twice the speed of sound. To date, the company has raised $41 million, which Scholl said will get the XB-1 airborne for the first time.