In the 1960s, Lockheed's secret Skunk Works developed the Mach 3 A-12 reconnaissance aircraft for the Central Intelligence Agency (CIA).

In October 1962, the CIA and the United States Air Force (USAF) instructed Lockheed to study a high-speed, high-altitude drone concept.

The Q-12 was to be air-launched from the back of an A-12, and used key technology from the A-12 project, including titanium construction and radar cross-section reduction design features.

[3] Johnson wanted to power the Q-12 with a ramjet engine built by the Marquardt Corporation for the Boeing CIM-10 Bomarc long-range surface-to-air missile.

[3] The camera and its film magazines with an inertial navigation system were carried in a cramped "Q-bay" below the drone's air intake.

The hatch would be ejected at the end of the mission and then snagged out of the air by a JC-130 Hercules, a technique that had been developed by the USAF to recover film canisters from balloons and satellites.

The M-21 was a two-seat version of the A-12, with a pylon on the fuselage centerline between the vertical stabilizers to carry the drone in a nose-up attitude.

Aerodynamic covers were initially placed over the D-21's intake and exhaust to reduce drag, but had to be removed after the first few tests, as no way of discarding them at Mach 3 without damaging the drone or carrier plane could be devised.

[9] A second launch took place on 27 April 1966; the D-21 reached its operational altitude of 90,000 ft (27,000 m) and speed of over Mach 3.3 (2,200 mph; 3,600 km/h), but was lost due to a hydraulic pump failure after a flight of over 1,200 nmi (1,400 mi; 2,200 km).

[10][12] Following the accident, Johnson suggested launching the D-21 from the Boeing B-52 Stratofortress bomber and adding a solid rocket booster to get it up to speed.

Command and telemetry systems were added, and high-speed cameras were installed to track the drones as they separated from the pylons.

[14] The solid-propellant rocket booster was both larger and heavier than the drone; it was 44 feet 4 inches (14 m) long and weighed 13,286 pounds (6,000 kg).

[15] During ground handling everyone within 25 feet (7.6 m) was required to wear anti-static straps to prevent any discharge of static electricity that might ignite the booster.

On 16 June the D-21B finally made a completely successful flight; it flew at the specified altitude and course for its full range, and the hatch was recovered.

The D-21B reached Lop Nor and returned to the recovery point, but the hatch had a partial parachute failure and was lost at sea with its photographs.

[24] On 23 July 1971, the D-21B program was canceled due to its poor success rate, the introduction of a new generation of photo reconnaissance satellites, and President Richard Nixon's rapprochement with China.

[27] The fate of the D-21 that had disappeared on the first operational flight was revealed in February 1986 when an official from the CIA returned a panel to Ben Rich that he had been given by a Soviet KGB agent.

[28] The Tupolev design bureau reverse-engineered the wreck and produced plans for a Soviet copy, named the Voron (Raven), but it was never built.

[29] In the late 1990s NASA considered using a D-21 to test a hybrid rocket-based combined cycle engine, which operates as a ramjet or rocket, depending on its flight regime.