Lockheed F-117 Nighthawk Have Blue

In 1975 DARPA informally invited Lockheed, Northrop and McDonnell Douglas to develop an aircraft under the name "Experimental Survivable Testbed" (XST), aimed at identifying the thresholds at which an aircraft became effectively undetectable by radar. Lockheed was not among the formally invited competitors at first — three of the original entrants dropped out early — and joined later alongside Northrop. Skunk Works engineer Ben Rich pushed the project internally after demonstrating, by applying radar cross-section calculations derived from the work of Soviet mathematician Pyotr Ufimtsev to a faceted airframe, that Lockheed could outperform its rivals in low observability.

The success of those preliminary studies led DARPA to expand funding and formally invite Lockheed's Skunk Works, in 1976, to build and test two subscale stealth fighters under the code name "Have Blue." These subscale aircraft incorporated jet engines from the Northrop T-38A, the fly-by-wire flight control system of the General Dynamics F-16, and the landing gear of the A-10, cutting development cost and risk. Although superficially similar to the later F-117, the Have Blue prototypes were considerably smaller — about a quarter of the F-117's weight — with a 72.5° wing sweep and inward-canted vertical tails (an inverse-V tail), features that were not carried over to the operational aircraft.

Construction of both demonstrators reused leftover tooling from the C-5 program, and final assembly was completed in a matter of weeks despite the strict secrecy surrounding the project: tarpaulins were thrown over the aircraft during outdoor engine runs to keep it from prying eyes, and the first prototype, HB1001, received an iron-ball radar-absorbent coating and a three-tone camouflage scheme devised by chief engineer Alan Brown to make casual identification harder. The demonstrators' maiden flight took place on 1 December 1977 at Groom Lake, Nevada, and the program's two test pilots alternated between flying the demonstrator itself and flying chase in the accompanying aircraft during each other's test flights.

Although both prototypes were lost to mechanical failures during the demonstration program, the flight data gathered fully validated the predicted radar cross-section and handling characteristics, and Have Blue was judged a success. That result led the U.S. government to increase funding for stealth technology and to order, under the code name Senior Trend, the development of an operational stealth fighter — the Lockheed F-117 — with the production contract awarded on 1 November 1978. Have Blue's technical legacy also carried over into the Senior Prom stealth cruise-missile demonstrator, derived from the same program and likewise flown at Groom Lake.