Sponsored by the USAF, the X-21A was designed to explore the feasibility of utilizing full-scale boundary layer control on a large aircraft. Paper and wind tunnel studies conducted by Northrop had indicated boundary layer control would offer numerous performance benefits. After successfully demonstrating the ability to achieve laminar flow over approximately 75 percent of the wing surface, the X-21As were used to explore the impact of rain, sleet, snow, and other weather anomalies on the system.
The two X-21As were modified from Douglas WB-66D Destroyer light bombers that had been retired from active service, and were equipped with a completely new wing and engine nacelles that were hung on either side of the aft fuselage.
First flown on 18 April 1963, the X-21As demonstrated that the boundary layer control technique, called laminar flow control, was both effective and viable. However, they also showed that these benefits came at a significant maintenance penalty as the numerous small slots required for the airflow constantly plugged up.
Last flown in 1964, the highest speed achieved was 560 mph (approx) and an altitude of 42,500 feet (approx).
Both X-21As survived the flight test program and were left in a bad state of repair on the photo range at Edwards AFB.