Continuing the Boeing never-built series with their largest 747 proposals: the 747-500 and 747-600:
Boeing announced the 747-500X and -600X at the 1996 Farnborough Airshow. The proposed models would have combined the 747's fuselage with a new 251 ft (77 m) span wing derived from the 777. Other changes included adding more powerful engines and increasing the number of tires from two to four on the nose landing gear and from 16 to 20 on the main landing gear.
The 747-500X concept featured an increased fuselage length of 18 ft (5.5 m) to 250 ft (76.2 m) long, and the aircraft was to carry 462 passengers over a range up to 8,700 nautical miles (10,000 mi, 16,100 km), with a gross weight of over 1.0 Mlb (450 tonnes). The 747-600X concept featured a greater stretch to 279 ft (85 m) with seating for 548 passengers, a range of up to 7,700 nmi (8,900 mi, 14,300 km), and a gross weight of 1.2 Mlb (540 tonnes). The cost of the changes from previous 747 models, in particular the new wing for the 747-500X and -600X, was estimated to be more than US$5 billion. Boeing was not able to attract enough interest to launch the aircraft.