A B747-500? Probably not.

The latest issue of Flight International (the Airline Directory M-Z issue) has an article on the Sonic Cruiser which reveals why Boeing has all but opted to go that way rather than compete with Airbus head-on. It also includes an editorial on this very subject.

The A380 is an idea whos time has come, and Airbus will finally move out from under the shadow of Boeing. The A380 is gaining momentum, and has captured the imagination of the airlines, because it is finally going to offer the economies of scale hoped for for so long. Airport and airway congestion are very serious issues, and this aircraft addresses that head-on.
Boeing market forcasts predicted that there would only be room for one such type in the near-term, and the way the order split has gone between the 747X and the A380 confirms Boeing’s market predictions. One can see in hindsight that the development of the 747X has been seen to be a ‘half-hearted’ effort, and the reality is that the 747 is perceived by the airlines to be a very old design which shows when compared to the A380.

Boeing would refuse to accept being eclipsed by its rival without a fight, and the Sonic Cruiser will provide the weapon for that fight. Boeing has said that it will NOT be supersonic; that it will in fact be designed to fly as close to the sound barrier as possible, circa Mach .98. Boeing has said that the fuselage cross section size will fit somehwere between the 777 and 767. This, in my opinion, is a mistake. I believe it should share as much commonality with the 777 as possible, including its cross section, to reduce development costs.