The Boeing AH-64 Apache is a four-blade, twin-engine attack helicopter with a tailwheel-type landing gear arrangement, and a tandem cockpit for a two-man crew. It features a nose-mounted sensor suite for target acquisition and night vision systems. It is armed with a 30 mm (1.18 in) M230 Chain Gun carried between the main landing gear, under the aircraft's forward fuselage. It has four hardpoints mounted on stub-wing pylons, typically carrying a mixture of AGM-114 Hellfire missiles and Hydra 70 rocket pods. The AH-64 has a large amount of systems redundancy to improve combat survivability. Development: Following the cancellation of the AH-56 Cheyenne in 1972, in favor of U.S. Air Force and Marine Corps projects like the A-10 Thunderbolt II and Harrier, the United States Army sought an aircraft to fill an anti-armor attack role that would still be under Army command;[6][7] the 1948 Key West Agreement forbade the Army from owning combat fixed-wing aircraft. The Army wanted an aircraft better than the AH-1 Cobra in firepower, performance and range. It would have the maneuverability for terrain following nap-of-the-earth (NoE) flying.[8] To this end, the U.S. Army issued a Request For Proposals (RFP) for an Advanced Attack Helicopter (AAH) on 15 November 1972.[9][10] As a sign of the importance of this project, in September 1973 the Army designated its five most important projects, the "Big Five" with AAH included. Proposals were submitted by Bell, Boeing Vertol/Grumman team, Hughes, Lockheed, and Sikorsky. In July 1973, the U.S. Department of Defense selected finalists Bell and Hughes Aircraft's Toolco Aircraft Division (later Hughes Helicopters). This began the phase 1 of the competition.[12] Each company built prototype helicopters and went through a flight test program. Hughes' Model 77/YAH-64A prototype first flew on 30 September 1975, while Bell's Model 409/YAH-63A prototype first flew on 1 October 1975. After evaluating the test results, the Army selected Hughes' YAH-64A over Bell's YAH-63A in 1976. Reasons for selecting the YAH-64A included its more damage tolerant four-blade main rotor and the instability of the YAH-63's tricycle landing gear arrangement.[13][14] The AH-64A then entered phase 2 of the AAH program under which three pre-production AH-64s would be built, additionally, the two YAH-64A flight prototypes and the ground test unit were upgraded to the same standard.[13] Weapons and sensor systems were integrated and tested during this time, including the laser-guided AGM-114 Hellfire missile.[15] Development of the Hellfire missile had begun in 1974, originally known by the name of Helicopter Launched, Fire and Forget Missile ('Hellfire' being a shortened acronym),[16] for the purpose of arming helicopter platforms with an effective anti-tank missile. Into production: In 1981, three pre-production AH-64As were handed over to the U.S. Army for Operational Test II. The Army testing was successful, but afterward it was decided to upgrade to the more powerful T700-GE-701 version of engine, rated at 1,690 shp (1,260 kW). The AH-64 was named the Apache in late 1981, keeping with the Army's traditional use of American Indian tribal names for its helicopters and it was approved for full scale production in 1982.[19] In 1983, the first production helicopter was rolled out at Hughes Helicopter's facility at Mesa, Arizona. Hughes Helicopters was purchased by McDonnell Douglas for $470 million in 1984.[20] The helicopter unit later became part of The Boeing Company with the merger of Boeing and McDonnell Douglas in August 1997.[21] In 1986, the incremental or flyaway cost for the AH-64A was $7M and the average unit cost was approximately $13.9M based on total costs.