How they built the Blackbird: Fascinating pictures show birth of Lockheed's top secret SR-71 spy plane

The Daily Mail has published a photo series showing how engineers in 1960s California secretly developed the SR-71 Blackbird for the US Air Force – a plane which still holds the record for being the fastest ever aircraft.

The Lockheed SR-71 Blackbird is a long-range, strategic reconnaissance aircraft and these once classified photos reveal how Lockheed built three types of Blackbird - A, B and C in top secret conditions in Burbank, California.

The aircraft were built by American aerospace company Lockheed under a 'black project' - a highly classified military defence project unacknowledged by the government until its declassification.

Since July 28 1976 the SR-71 Blackbird has held the world record for the fastest air-breathing manned aircraft with a record speed of 2,193.2mph.

Renowned American aerospace engineer Clarence 'Kelly' Johnson was responsible for many of the design's innovative concepts.

During reconnaissance missions, the SR-71 operated at high speeds and altitudes to allow it to outrace threats - if a surface-to-air missile launch was detected, the standard evasive action was simply to accelerate and out fly the missile.

The SR-71 was designed to have basic stealthy characteristics and served as a precursor to modern stealth aircraft.

The Blackbird served with the U.S. Air Force from 1964 to 1998 and a total of 32 aircraft were built; 12 were lost in accidents, but none were ever lost to enemy action.