The quadjet 747 uses a double-deck configuration for part of its length and is available in passenger, freighter, and other versions. Boeing designed the 747’s hump-like upper deck to serve as a first-class lounge or extra seating, and to allow the aircraft to be easily converted to a cargo carrier by removing seats and installing a front cargo door. Boeing expected supersonic airliners—the development of which was announced in the early 1960s—to render the 747 and other subsonic airliners obsolete, while the demand for subsonic cargo aircraft would remain robust well into the future. Though the 747 was expected to become obsolete after 400 were sold, production passed 1,000 in 1993. By June 2019, 1,554 aircraft had been built, with 20 of the 747-8 variants remaining on order. As of January 2017[update], 60 of the jets have been lost in accidents in which a total of 3,722 people died.