Stealth aircraft are designed to avoid detection using a variety of technologies that reduce reflection/emission of radar, infrared, visible light, radio -frequency (RF) spectrum, and audio, collectively known as stealth technology. Development of stealth technology likely began in Nazi Germany during World War II with the prototype Horten Ho 229 as the first stealth aircraft. Well-known modern examples of stealth of U.S. aircraft include the United States ' F-117 Nighthawk (1981–2008), the B-2 Spirit, the F-22 Raptor, and the F-35 Lightning II.
While no aircraft is totally invisible to radar, stealth aircraft make it more difficult for conventional radar to detect or track the aircraft effectively, increasing the odds of an aircraft successfully avoiding detection by enemy radar and/or avoiding being successfully targeted by radar guided weapons. Stealth is the combination of passive low observable (LO) features and active emitters such as low-probability-of-intercept radars, radios and laser designators. These are usually combined with active measures such as carefully planning all mission maneuvers in order to minimize the aircraft 's radar cross section, since common actions such as hard turns or opening bomb bay doors can more than double an otherwise stealthy aircraft 's radar return. It is accomplished by using a complex design philosophy to reduce the ability of an opponent's sensors to detect, track, or attack the stealth aircraft. This philosophy also takes into account the heat, sound, and other emissions of the aircraft as these can also be used to locate it.
Full-size stealth combat aircraft demonstrators have been flown by the United States (in 1977), Russia (in 2010) and China (in 2011). As of March 2017, the United States Armed Forces utilize three models of dedicated, manned stealth aircraft and the Chinese Air Force operates one, with a number of other countries developing their own designs. There are also various aircraft with reduced detectability, either unintentionally or as a secondary feature.
