With its bizarrely irregular shape and jet-black color, the Lockheed Martin F-117A Nighthawk looks more like Darth Vader's personal spacecraft, than an aircraft that is part of the U.S. Air Force fleet. But the F-117A, which employs revolutionary stealth technology to fool enemy radar, is a crucial part of the U.S. arsenal. For much of its quarter-century of existence, the stealth fighter-bomber operated behind such a clandestine veil that some government officials are said to have suspected that it was actually a time machine. In recent years, however, enough details about the F-117A's long development and changing mission have emerged. Here is a brief history of this remarkable aircraft.

How to Hide in Plain Sight
During World War I, military aviators in Germany and Great Britain sought to make their early fighters and bombers invisible to the naked eye, according to military historian Allan D. English. One particularly imaginative inventor actually tried building a warplane with clear cellophane skin. (Unfortunately for him, the cellophane reflected sunlight, making it even more visible from a distance.)

But when radar was invented just prior to World War II, emphasis shifted toward evading electronic detection. During the aerial Battle of Britain in 1940, the British and Germans both tried jamming each others' radar systems, but were easily able to counter each others' electronic trickery. The British, however, got the idea of creating an aircraft that was more difficult for radar to spot. They built the De Havilland Mosquito, a bomber with a wooden exterior instead of metal, which made the aircraft reflect less of the radiation that radar systems bounce off objects to identify them. (It was an early use of what engineers call RAM, or radar-absorbing material.)

After the war, British and American aircraft manufacturers continued to search for a way to achieve a low radar cross section, or RCS, the measure of a plane's visibility to radar. Both experimented with "flying wing" designs that would be less visible on radar screens. In the 1950s, the Americans developed the U2 and SR-71 reconnaissance aircraft, which utilized plastic honeycomb in the wings and were coated with a special iron-based paint with RAM properties.

In the 1970s, a key piece of the stealth formula emerged when the Americans discovered an obscure 20-year-old scientific paper by Pyotr Ufimtsev, a Russian scientist who had proposed reducing RCS by building an aircraft with flat surfaces and irregular angles, rather than the usual aerodynamically streamlined profile. Bill Schroeder, a mathematician for aircraft maker Lockheed (now Lockheed Martin), developed a computer program that made it possible to predict precisely the RCS of such an aircraft exterior.

Eventually, Schroeder's methodology led to a design in which the exterior's facets focused incoming radar radiation into narrow beams and redirected the beams away from the enemy's radar equipment. Designers jokingly called it "the hopeless diamond," because its shape was less than ideal for flying, but stealth was on its way to becoming a reality.