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.

The effort to develop stealth technology by Lockheed and Northrup in the 1970s and 1980s was one of the government's most tightly guarded secrets. Nevertheless, enough stray details leaked out to fuel the imaginations of investigative reporters, UFO buffs, and conspiracy theorists, who sometimes got the information wrong in a way that only heightened the mystery. (According to aviation historian Robert Dorr, the very term "stealth," for example, came from press reports that garbled the word "survivable" in a top-secret project name.) In 1981, a Lockheed test pilot flew an early version of what became the F-117A at the Air Force test facility in Groom Lake, Nev., a remote site with such tight security that any civilian pilot who strayed too close risked being shot down by anti-aircraft missiles. For the next few years, the research and development continued, initially at Groom Lake and later at the Tonopah Test Range northwest of Las Vegas, a place so shrouded in secrecy that workers were housed 200 miles away, and a palm print was required for admittance to the facility's inner grounds.

In creating a radar-proof aircraft, designers found they had to make compromises. The F-117A was slower than other aircraft with similarly sized engines because of its less aerodynamic shape, and was less stable (hence its nickname, the "Wobblin' Goblin"). The aircraft couldn't use radar, lest it give signals that would betray its identity, and it couldn't carry any bombs or missiles attached to its underside, because that would have created a large radar signature. But American scientists and engineers developed other technological advances, such as lightweight carbon-based materials to keep the oddly-shaped aircraft from being too heavy, and infrared instruments that allowed it to operate in darkness.

Pentagon leaders decided not to use the F-117A against Libya in 1986, in part because they were reluctant to reveal its existence to the world. The government unveiled the aircraft in 1988, and two years later, it was used in the invasion of Panama.

It was not until the 1991 Gulf War, however, that the F-117A flew an extensive number of missions. As designers had hoped, the stealth fighter-bomber slipped through radar like Gene Kelly dancing through raindrops, so bewildering Iraqi anti-aircraft units that they resorted to simply firing wildly into the sky. Not a single aircraft was lost.

In 1999, F-117As were used again in Yugoslavia, as part of NATO's military effort to compel the regime of Slobodan Milosevic to cease its war in Kosovo and allow international peacekeepers and hundreds of thousands of refugees to return to the embattled region. One aircraft was shot down by the Serbians — apparently by sheer luck, rather than the use of radar —but the American pilot was rescued.

Military historian Allan D. English, author of the 1998 book The Changing Face of War, describes stealth technology as the 21st-century equivalent of the crossbow, musket, tank or nuclear submarine. Just as each of those technologies radically altered military strategy and tactics in their time, English and other scholars believe, so has stealth transformed the nature of war.

So far, no other nation has developed stealth technology comparable to the F-117A, but with the aircraft steadily aging — the last ones rolled off the assembly line nearly 15 years ago — there's pressure to find a replacement. Over the years, rumors of other successors have surfaced — mostly just their mysterious-sounding names, such as "Black Manta," "Aurora" and "Mothership." Meanwhile, undoubtedly, other nations continue to try to find ways to detect the F-117A. Russian fighters, for example, reportedly carry devices that attempt to track the stealth fighter-bomber's heat. So far, however, the aircraft has been tested against upward of 40 different surveillance technologies, and remained invisible to them all.