In the 1970s the CIA got a copy of Prof. Ufimtsev's work on radar diffraction through use of geometric shapes out of the USSR. The work became the core of a plan to develop what was called "The Hopeless Diamond" which became Lockheed's XST under the "Have Blue" codename. This was the first test of faceting with the predecessor of the F-117. The XST was lighter, less complicated, had a greater degree of wingsweep at 72.5o, an inward-canted tails mounted further forward, and having no bomb bay nor tactical sensors mounted. It was cobbled together using A-10 landing gear, T-28 engine, and F-5 instrumentation and ejection seat. Only capable of 600 mph it was used to test the RCS of the aircraft while in flight. On the ground it was covered in splinter cammo to hide the facets. Two examples were built and flown. One crashed on its 39th flight and the second on its 52nd. Both were buried to protect their secrets. Everything learned from the XSTs was modified into the "Senior Trend" project for a combat stealth aircraft that would become the F-117 Nighthawk by 1981. Northrop also had a XST but it was judged inferior b/c Northrop put too much RCS reduction emphasis on the frontal aspect of the aircraft rather than the entire airframe (see last pic below).

http://q-zon-fighterplanes.com/wp-content/uploads/2010/06/Lockheed-XST-Have-Blue.jpg http://www.aer.ita.br/~bmattos/mundo/country/usa/usa/lok_xst.jpg http://www.hitechweb.genezis.eu/stealth2.files/lockheed_have_blue_7.jpg Northrop XST: http://www.hitechweb.genezis.eu/stealth2.files/northrop_xst_01.jpg