Modern airliners are usually low-wing designs with engines mounted in pods under the wing (usually two of them). For airliners, multi-engine design is mandated by regulations so that the plane can continue to climb and fly even in the case of power loss in engines right after take-off. Another regulatory demand is that aircraft are able to fly a minimum specified amount of time after one engine fails in flight. (Right): A Boeing 747 in flight. As can be see, the 747 follows the conventional underwing engine layout, employing 4 such engines with two under each wing.
Mounting the engines underneath and to the fore of the wing moves weight from the fuselage to the wings, imposing less bending movement on them and allowing for a lighter wing structure. After this feature proved successful in military jets, Boeing introduced it to its 707 airliner design and it has been increasingly adopted since.
Mounting the engines in underwing pods also makes access for maintenance quicker and easier compared to tail-mounted engines. Additionally, low wing design helps keep the engine nacelles and refuelling valves closer to the ground to simplify access. Also, the wing's surface acts as a barrier to prevent the engines' noise from reaching the passenger cabin.
Almost all major aircraft manufacturing companies follow the engine-underneath-the-wing template, although Bombardier Learjet is one of the few who follow the engine-on-the-tail scheme. The Learjet 60 is one of their planes which has its engines mounted on its tail. (Above): The Learjet 60, one of the few planes which incorporates tail-mounted engines.
Future airliners may feature delta wing designs or the blended wing body design (BWB), a design where the whole of the aircraft is one structure and has no distinguishable fuselage. The Boeing X-48 is Boeing's experimental craft to test such innovative designs. (Right): The Boeing X-48 Blended Wing Body experimental craft.