The American designs, the "SST" project (for Supersonic Transport) were the Boeing 2707 and the Lockheed L-2000. These were to have been larger, with seating for up to 300 people.[290][291] Running a few years behind Concorde, the Boeing 2707 was redesigned to a cropped delta layout; the extra cost of these changes helped to kill the project.[292] The operation of US military aircraft such as the Mach 3+ North American XB-70 Valkyrie prototypes and Convair B-58 Hustler strategic nuclear bomber had shown that sonic booms were quite capable of reaching the ground,[293] and the experience from the Oklahoma City sonic boom tests led to the same environmental concerns that hindered the commercial success of Concorde. The American government cancelled its SST project in 1971, after having spent more than $1 billion.[294]