Boeing has not ruled out a further stretch of the 747 but any "-9" development will have to wait its turn in the product development cycle behind higher priority projects.

"The 747 could [be stretched], some day," says 747 programme manager Elizabeth Lund.

Speaking on board the 747-8I's first passenger service, flown by Lufthansa between Frankfurt and Washington Dulles on 1 June, Lund added: "Once in a while on the drawing board we toy with a few things, but it is not imminent. Boeing's product development priorities are to finish the 787-9, look at a 787-10X, the 737 Max and what to do with the 777.

"So the 747 [has to wait its] turn on the product development board - we need to wrap up and fly for a little while, while the other models are developed."

Compared with the 747-400, the General Electric GEnx-powered 747-8I incorporates a 5.6m (18.3ft) stretch and modified wing, and an approximately 50-seat capacity increase to 467 passengers. The -8 also offers around 330nm (610km) more range than the 747-400ER.

Back in 1996, Boeing came close to launching a 550-seat "747-600" derivative which featured a stretch of around 14m over the -400, a larger wing and was powered by either Rolls-Royce Trent or Engine Alliance GP7000 engines. The project foundered after it was unable to secure launch orders.