<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by Gaucho100K:
There is obviously a limit on how much you can stretch these birds... Im assuming that the 747 fuselage is pretty much near its limit... if this were not the case, Boeing could have just added length to the 744 and killed the A380, or...?</font>
Boeing has actually considered this with the -500X at 250 feet long and the -600X at 279 feet long. The wingspan on both is 251 feet, which generally is the limiting factor. That's why the -400 has winglets, it allowed a shorter span, thereby allowing the plane to use existing airside facilities. Both versions involve fuselage stretches in front of and behind the wing, new wings, wing fairings, engines, stabilizers, elevators, and rudders.
They have also considered a completely double-deck 74. As far as I know, all these plans are currently on the shelf.