It rose from the Army's Advanced Aerial Fire Support System (AAFSS) program to field the service's first dedicated attack helicopter.

Lockheed designed the Cheyenne using a four-blade rigid-rotor system and configured the aircraft as a compound helicopter with low-mounted wings and a tail-mounted thrusting propeller driven by a General Electric T64 turboshaft engine.

In 1966, the Army awarded Lockheed a contract for ten AH-56 prototypes, but as a stopgap also ordered the less complex Bell AH-1G Cobra as an interim attack aircraft for combat in Vietnam War.

A fatal crash and technical problems affecting performance put the helicopter's development behind schedule, resulting in the cancellation of the production contract on 19 May 1969.

[6] The recommendation of the Howze Board came at the same time the Army was preparing to deploy its first armed escort helicopters to Vietnam; 15 UH-1A Iroquois were modified with systems for mounting machine guns, grenade launchers, and rocket pods.

The D-255 Iroquois Warrior was envisioned as a purpose-built attack aircraft based on the UH-1B airframe and dynamic components, with a nose-mounted ball turret, a belly-mounted gun pod, and stub wings for mounting rockets or SS.10 anti-tank missiles.

[8] In December 1962, Combat Development Command (CDC) drafted a Qualitative Material Requirement (QMR) for an interim, commercial off-the-shelf (COTS) aircraft, with a 140-knot (160 mph; 260 km/h) cruise speed and a 1,500-pound (680 kg) payload.

The Director of Defense Research and Engineering (DDRE) conditionally approved the changes to the development objectives, pending his review of the proposed program.

[9] As a result, the Army Material Command (AMC) conducted a study to determine if the development objectives were feasible and also established a program office for the Fire-support Aerial System (FAS).

[9] On 26 March 1964, the Army Chief of Staff redesignated the FAS program as the Advanced Aerial Fire Support System (AAFSS).

The development objectives document (QMDO) for the AAFSS was approved in April 1964, and on 1 August 1964, the Transportation Research and Engineering Command contacted 148 prospective contractors with a request for proposals (RFP).

Sikorsky submitted the S-66, which featured a "Rotorprop" that would serve as a tail rotor but as speeds increased would rotate 90° to act as pusher propeller.

The document added fourteen requirements that were not previously addressed by Lockheed's proposal, including the addition of an aerial rocket armament subsystem.

At high speeds, the amount of lift provided by the wings, along with thrust from the pusher prop, reduced the aerodynamic loading of the rotor.

[17] During early flight tests, a rotor instability issue was discovered when the aircraft was flying at low altitude in ground effect.

[28] The project suffered a setback on 12 March 1969, when the rotor on prototype #3 (s/n 66-8828) struck the fuselage and caused the aircraft to crash, killing the pilot, David A. Beil.

In response, Lockheed proposed an "improved flight control system" (ICS) to reduce rotor oscillations, and steps for removing excess weight and addressing other minor issues in production helicopters.

The engineers did not realize that the fixed mounts used to secure the aircraft in the wind tunnel would not allow the helicopter to move relative to the rotor, as it did in flight.

During high speed testing to replicate the half-P hop vibration, the rotor oscillations quickly accelerated out of control and struck the tail boom, resulting in the destructive breakup of the helicopter.

[34] Cheyenne prototype #6 (s/n 66-8831) began conducting weapons testing at Yuma Proving Ground, Arizona, demonstrating the ability for the gunner and pilot to accurately fire on separate targets on each side on the helicopter.

[35] Prototypes #6 and #9 were also tested and evaluated at Yuma Proving Ground from 30 January to 23 December 1971, to determine if stability and control systems were sufficient.

[34] Following the testing at Yuma, prototype #9 received the improved T64-GE-716 engine producing 4,275 shp (3,188 kW) and the planned production version of the ICS system.

[38] The Department of Defense (DOD) conducted a study that concluded that the Air Force's A-X program, the Marine Corps' Harrier, and the Cheyenne were significantly different that they did not constitute a duplication of capabilities.

[4] On 22 October 1971, the Senate Armed Services subcommittee on Tactical Air Power conducted hearings to evaluate the CAS mission and the pending programs.

[4] The task force conducted flight evaluations of the AH-56, along with two industry alternatives for comparison: the Bell 309 King Cobra and Sikorsky S-67 Blackhawk.

[4][8][40] The Army also conducted a weapons demonstration for the Senate Armed Services Committee in early 1972, to show off the Cheyenne's firepower and garner support for attack helicopter development.

The report recommended funding of the Air Force's A-X program, which would become the A-10 Thunderbolt II and limited procurement of the Harrier for the Marine Corps.

The report never referred to the Cheyenne by name and only offered a lukewarm recommendation for the Army to continue procurement of attack helicopters, so long as their survivability could be improved.

[43] AAH sought an attack helicopter based on combat experience in Vietnam, with a lower top speed of 145 kn (167 mph; 269 km/h) and twin engines for improved survivability.