The Convair X-12 was the second, more advanced testbed for the Atlas program. It was in contrast to its predecessor, the Convair X-11, which had only one engine, equipped with three engines. It was powered by a multi -stage rocket with liquid fuel. The first flight took place in July 1958.
The X-12 was the first aircraft with a 1.5 - stage rocket motor, which later became a hallmark of the Atlas program has been. She was after the Soviet R- 7, the first U.S. missile whose range could be designated as 10,179 km intercontinental.