Here is some history of the A-12.
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The A-12 is the forerunner of the SR-71 and has nearly the same shape and dimensions as its replacement. Designed to replace the U-2, the A-12 flew higher and four times as fast to outrun enemy defenses and gather intelligence. The A-12 is primarily an over flight vehicle unlike the SR-71. Its major advantages in capabilities to the SR-71 include its higher-resolution photography and its ability to go marginally faster (Mach 3.3) than the SR-71. However, the SR-71 was chosen as successor to the A-12 due to its side-looking radar and cameras, allowing it to gather important reconnaissance data without penetrating enemy airspace.
This is a photo of the A-12, from Lockheed.
Here is the historical timeline of the A-12
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24 December 1957: First J58 engine run.
21 April 1958: First mention of ARCHANGEL in Kelly Johnson's dairy.
December 1958: CIA requests funding for a Mach 3+ reconnaissance aircraft.
29 August 1959: Lockheed and Convair each propose a plan for a Mach 3+ reconnaissance aircraft.
14 September 1959: CIA awards first ARCHANGEL research contract to Lockheed.
26 January 1960: CIA orders twelve A-12 aircraft.
February 1960: Lockheed begins the search for 24 pilots for the A-12.
1 May 1960: Francis Gary Powers is shot down in a U-2 over the Soviet Union.
26 February 1962: First A-12 leaves Skunk Works in Burbank for Groom Lake by truck.
25 April 1962: First flight of the prototype A-12 (#06924) with Lockheed test pilot Lou Schalk.
30 April 1962: First "official" flight of A-12 (#06924) with Lockheed test pilot Lou Schalk.
2 May 1962: A-12 goes supersonic for first time during second test flight.
30 July 1962: J58 completes pre-flight testing.
5 October 1962: A-12 flies with J75 (in left nacelle) and J58 (in right nacelle) engines.
15 January 1963: A-12 first flight with two J58 engines.
24 May 1963: First A-12 crashes (#06926) near Wendover, UT.
20 July 1963: First A-12 flight over Mach 3.
November 1963: A-12 reaches design speed and altitude.
3 February 1964: A-12 cruises at Mach 3.2 and 83,000 feet for 10 minutes.
June 1964: Final A-12 (#06939) delivered to Groom Lake.
27 January 1965: A-12 flown for one hour and 40 minutes above Mach 3.1 for a distance of 3,000 miles.
28 December 1966: Decision is made to conclude A-12 operations by 1 June 1968.
22 May 1967: First A-12 (#06937) flown to Kadena AB by CIA pilot Mel Vojovodich.
29 May 1967: BLACK SHIELD unit declared operational at Kadena AB.
31 May 1967: First A-12 (#06937) operational mission over North Vietnam lasted 3 hours, 39 minutes.
3 November 1967: A-12 and SR-71 conduct a reconnaissance fly-off. Results were questionable.
23 January 1968: First A-12 overflight of North Korea during Pueblo incident with CIA pilot Frank Murray.
8 May 1968: Last A-12 operational mission flown.
5 June 1968: Last A-12 (#06932) to crash, lost in the South China Sea.