Today's airliners are a long tubular fuselage, carrying the passengers and cargo atop a wing that does all the aerodynamic work, lifts, stability etc. A good deal of sheet metal goes along just to carry the payload. A more efficient design (illustrated on the cover of Aviation Week) blends the wing and the fuselage into a single body, like the B2 flying wing bomber. The B2 is as efficient as it gets, it's all wing, no structure is just along for the ride. Fortunately the payload (iron bombs) is good and dense and doesn't take up much room inside the wing. Passengers are not that dense.
So the blended wing Lockheed design is a wing with a great swelling in the middle to accept a passenger cabin. Trouble is, cabins have to be pressurized, which imposes enormous forces trying to blow the cabin open. With only 5 pounds per square inch cabin pressure, over the 13 million square inches of a typical cabin, you get nearly 70 million pounds of force straining the cabin walls. The only structure that can resist this is a round tube, like present day airliner fuselages. So the Lockheed designers have a cylindrical passenger cabin buried inside their swoopy blended wing/body swelling. Trouble is, we have many feet of space between the cabin wall and the outer skin. Which makes cabin windows impossible. Which doesn't bother the designers, cabin windows are a pain, heavy, prone to leaks, points of weakness, and crack start locations. They are happy to omit cabin windows.
Passengers are not in favor. They like window seats, they like being able to see out, and they like sunshine. Boarding a windowless airliner gives some of them the willies, and depresses many others.
Maybe the conventional jet liner design is not so inefficient after all.