Torrejón Air Base was originally the home of the Spanish National Institute of Aeronautics, however following the Pact of Madrid of 26 September 1953, construction began at Torrejón on a new concrete runway to replace the existing grass airstrip and on a massive concrete apron and other necessary maintenance and shelter facilities to accommodate the largest United States Air Force bomber aircraft in the Strategic Air Command (SAC) inventory, with the base intended to support SAC Reflex missions. This was the longest runway in Europe until the construction of the Ulyanovsk Vostochny Airport in 1983, and is the ninth longest runway in Europe as of 1 June 2017.