Ever since the last flight of the Concorde, the priorities for air travel have changed from going faster to going more efficiently. An EU-sponsored study called LAPCAT has been trying to shake things up with a hypersonic aircraft called the A2, which would take 300 passengers anywhere on Earth at over Mach 5.
If this concept design looks a little bit familiar, it should: the company behind it is Reaction Engines, who've also been working on the Skylon spaceplane. While the A2 won't be going to orbit, it's still going to use the same primary engines as the Skylon, and the fuselage design is similar so as to minimize drag and friction heating at hypersonic speeds. 300 passengers should be able to fit inside the A2, and all of them will likely be paying a speed premium akin to the cost of a business class ticket on any other airline. What they won't be paying for are windows, since anything that could withstand the 1,800 degree air temperatures at Mach 5 would be prohibitively heavy. Expect flatscreen displays instead.
The engines are of course the trickiest part of this whole hypersonic endeavor. There are two issues to consider: thrust, and heat. Obviously, thrust is very important, but at high Mach numbers the heat from the incoming compressed air can cause engines to melt if too much fuel is added. The solution that Reaction Engines has come up with is to use liquid hydrogen as fuel, and pass incoming air around a heat exchanger with fuel inside. The air cools down while the fuel heats up, but the fuel doesn't mind since it gets burned up shortly thereafter anyway. With a system like this, the A2 should have a cruise speed of Mach 5.2 (about 4,000 mph) and a range of over 12,000 miles on 200 tons of liquid hydrogen.
Part of the reason that LAPCAT (which stands for Long-Term Advanced Propulsion Concepts and Technologies) is looking at a 25 year time frame for the A2 is that we don't have a good way of efficiently creating lots of liquid hydrogen. There are other issues as well (like the fact that the program will run out of money in 2013), but the LAPCAT consortium is optimistic that they'll be able to convince the EU and private investors that the A2 is worth pursuing, and if they can do it, expect a first flight sometime around 2040.