Boeing 747-451 N401PW, the prototype 747-400, at the manufacturer's Everett plant near Seattle during the test phase in 1988. Northwest Airlines later took delivery of this aircraft as the launch customer in December the following year, registering it as N661US. Surviving the merger with Delta, N661US was finally retired after nearly 26 years of commercial service in September 2015. (Photo via Instagram: Boeing)
