Four test vehicles were planned, but only two were built, both atmospheric test vehicles. The airframes, which were largely built of composite materials, were constructed under contract by Scaled Composites. The first flew its maiden flight on March 12, 1998. The X-38 utilized a unique parafoil landing system designed by Pioneer Aerospace. The ram-air inflated parafoil used in the flight test program was the largest in the world, with a surface area of . The parafoil was actively controlled by an onboard guidance system that was based on GPS navigation.