Designed as a testbed for technologies being developed for the Lockheed A-12, the Lockheed A-10-4 has often been recorded as merely a modified F-104. The 10-4 was however, almost a completely new aircraft.
Instrumental in the development of the bleed-bypass engine (albeit using a modified P & W J75 rather than the J58) as well as pressurisation and fuel systems for the A-12, the A-10-4 was a real hotrod but lacked range when compared to it's later siblings.
Lockheed A-10-4 USAF by
mtpalmer1, on Flickr
Note the wingtip tanks which were jettisoned at the top of the climb for high-speed flight.