The German Ruhrstahl X-4 was an air to air missile, the first of its kind in the world. As Germany came under constant bomber attack, the Luftwaffe could not keep up with the amount of bombers the Allies put up. They developed many weapons specifically designed to kill bombers such as the MK 108 30mm cannon among many other things, however these required fighters to get close in and attack the bombers formations where they could be engaged by them as well. A German researcher named Max Kramer began work on the Ruhrstahl X-4 air to air missile as an answer to the bomber problem. It would be a wire guided missile that could be fired from up to 3 miles away, allowing the attacking aircraft to do so from a safe distance. However, because of the difficulty in guiding the weapon it was also fixed with an acoustic trigger, much akin to a torpedo using acoustics to help guide it on target or tell it to explode. Once the missile reached a certain range to the B-17 it could detect its sound and would trigger it to explode. The X-4 carried a 20Kg payload which could severely damage or outright knock a B-17 out of the sky. The X-4 was designed to be launched from single engine fighters, however as tests were carried out on the Fockewulfe 190 fighter it was found to be to difficult to guide and fly at the same time. The Germans decided that multi crew planes were the answer to the problem, and the JU-88 and ME-262 were used. Strangely enough the ME-262 was usually a 1 seater fighter jet aircraft but I suppose they would be using the fighter bomber version which did house 2 crew. The plane specifically designed to use this weapon never got off the drawing board, it was called the Focke-Wulf Ta-183 (picture [url]http://upload.wikimedia.org/wikipedia/commons/a/aa/Ta_183_Modell.jpg[/url] that is a model of what the plane would have looked like, however you don't have to look far for similarities in other countries aircraft on that design, the Russian Mig-15 looks exactly like it! As with every German Wunderwaffe or experimental weapon enough time was never devoted to it, or it simply arrived to late. The Ruhrstahl X-4 has unconfirmed use over the skies of Germany in 1945 but it was never delivered to the Luftwaffe apparently. It went on to influence the development of Allied and Warsaw pact ATGM's (Anti tank guided missiles, which used the same guidance MACLOS system that the X-4 used, some still used today like the NATO TOW system) and air to air missiles. The X-4 also had a similar design used as an anti tank missile, which never saw combat at all (It is same to assume this version was also used by other countries after the war in there development of ATGM's) . The X-4 has radically changed warfare from World War 2 to now even if it may have never been used.