Succeeded by it's younger Blackbird counterpart, the A-12 Oxcart was Lockheed's response to the U.S. Air Force's (USAF) need for a high-altitude supersonic strategic reconnaissance aircraft.  Smaller, faster, more stealthy, and agile than it's younger counterpart, the A-12 conducted a field of operations over Laos, Cambodia, Vietnam, and the Soviet bloc.  So secretive was it's existence that it was hiding almost in plain sight.  To cover up the A-12 program, the USAF posited that the aircraft was a high-speed interceptor known as the YF-12.  The YF-12 variant featured a nose cone juxtaposed to the tapered design in the Blackbird and the Oxcart.  

The existence of the A-12 and it's mission set was not revealed to the public until after the turn of the twenty-first century - over thirty years since it had last flown.

The nomenclature for the A-12 can be traced back to it's developmental history.  A formal designation was never given; likely due to its secrecy.  The "A" referred to Lockheed's program that aimed to replace the U-2 Dragon Lady; Archangel.  The Archangel program had twelve different designs within it that were presented as feasible replacements between 1956 and 1959.  The produced design was number twelve, hence the "-12" designation.

Of the fifteen aircraft produced, only nine remain with the remainder lost.