Question: The American Aircraft Penetrator was modified from a helicopter produced by what nation's military?

full_context: Document 1: American Aircraft Penetrator: The American Aircraft Penetrator, now referred as the Aerocraft Stealth Star 204 SS, was a gunship helicopter modified from the Bell UH-1B Iroquois with tandem seating for the pilots and a troop-carrying compartment.
Document 2: List of surviving Sikorsky CH-54s: The Sikorsky CH-54 Tarhe is a twin-engine heavy-lift helicopter designed by Sikorsky Aircraft for the United States Army. The civil version is the Sikorsky S-64 Skycrane. The Army purchased 105 CH-54s before its discontinuation. The S-64 Aircrane is still in production. There are currently numerous complete and partial airframes in existence for this aircraft. Some CH-54's and some S-64's (both E and F models) are owned, and have been remanufactured, by Erickson Air-Crane Inc."
Document 3: Bell AH-1Z Viper: The Bell AH-1Z Viper is an American twin-engine attack helicopter, based on the AH-1W SuperCobra, that was developed for the United States Marine Corps as part of the H-1 upgrade program. The AH-1Z features a four-blade, bearingless, composite main rotor system, uprated transmission, and a new target sighting system. The AH-1Z, one of the latest members of the prolific Huey family, is also called "Zulu Cobra", based on the military phonetic alphabet pronunciation of its variant letter.
Document 4: AAI Penetrator: The American Aircraft International Penetrator was a military helicopter prototype conceived in 1990 as a gunship conversion of the Bell UH-1 Iroquois, aimed at ground insertion, close support and counter-insurgency roles.
Document 5: Northrop Grumman B-2 Spirit: The Northrop (later Northrop Grumman) B-2 Spirit, also known as the Stealth Bomber, is an American heavy penetration strategic bomber, featuring low observable stealth technology designed for penetrating dense anti-aircraft defenses; it is a flying wing design with a crew of two. The bomber can deploy both conventional and thermonuclear weapons, such as eighty 500 lb -class (Mk 82) JDAM Global Positioning System-guided bombs, or sixteen 2400 lb B83 nuclear bombs. The B-2 is the only acknowledged aircraft that can carry large air-to-surface standoff weapons in a stealth configuration.
Document 6: Harbin Z-20: The Harbin Z-20 or Zhi-20 is a medium lift helicopter produced in the Northeast of China. Its first flight was on 23 December 2013. The helicopter has a maximum takeoff weight in the range of 10 tons, can drop troops at locations of up to 3,000 ft altitude, and could operate from the Chinese aircraft carrier Liaoning. It is thought to be comparable to the US made Sikorsky UH-60 Black Hawk helicopter, the civilian Sikorsky S-70C-2 variant of which has been used by the People's Liberation Army since 1984. Some sources suggest that the Z-20 is a copy of the Black Hawk and link the design to the Black Hawk that was abandoned by US special forces in Pakistan during the operation to kill Osama bin Laden on 1 May 2011. The sources say that Pakistan allowed Chinese officials to examine the Black Hawk wreckage. However, Aviation Week also points out that although some aspects of the design do appear similar, such as the tail wheel arrangement, there are also marked differences. For example, the Chinese Z-20 has a five blade rotor compared with the Black Hawks' four blades.
Document 7: Gunship (video game): Gunship is an AH-64 Apache helicopter combat flight simulator video game that was developed and released by MicroProse in 1986. Simulating a helicopter gunship, players navigate through missions to attack enemy targets and protect friendly forces.
Document 8: Bell UH-1 Iroquois: The Bell UH-1 Iroquois (nicknamed "Huey") is a utility military helicopter powered by a single turboshaft engine, with two-blade main and tail rotors. The first member of the prolific Huey family, it was developed by Bell Helicopter to meet a United States Army's 1952 requirement for a medical evacuation and utility helicopter, and first flew in 1956. The UH-1 was the first turbine-powered helicopter to enter production in 1960 for the United States military, and more than 16,000 have been built since.
Document 9: Vought-Sikorsky VS-300: The Vought-Sikorsky VS-300 (or S-46) was a single-engine helicopter designed by Igor Sikorsky. It had a single three-blade rotor originally powered by a 75 horsepower (56 kW) engine. The first "free" flight of the VS-300 was on 13 May 1940. The VS-300 was the first successful single lifting rotor helicopter in the United States and the first successful helicopter to use a single vertical-plane tail rotor configuration for antitorque. With floats attached, it became the first practical amphibious helicopter.
Document 10: Bell AH-1 SuperCobra: The Bell AH-1 SuperCobra is a twin-engine attack helicopter based on the United States Army's single-engine AH-1 Cobra. The twin Cobra family, itself part of the larger Huey family, includes the AH-1J SeaCobra, the AH-1T Improved SeaCobra, and the AH-1W SuperCobra. The AH-1W, the backbone of the United States Marine Corps's attack helicopter fleet for decades is being replaced by the next generation Bell AH-1Z Viper attack helicopter.
Document 11: Sikorsky R-4: The Sikorsky R-4 was a two-seat helicopter designed by Igor Sikorsky with a single, three-bladed main rotor and powered by a radial engine. The R-4 was the world's first large-scale mass-produced helicopter and the first helicopter used by the United States Army Air Forces, the United States Navy, the United States Coast Guard and the United Kingdom's Royal Air Force and Royal Navy. In U.S. Navy and U.S. Coast Guard service, the helicopter was known as the Sikorsky HNS-1. In British service, it was known as the Hoverfly.
Document 12: Sikorsky S-67 Blackhawk: The Sikorsky S-67 Blackhawk was a private-venture, prototype attack helicopter built in 1970 with Sikorsky Aircraft research and development (R&D) funds. A tandem, two-seat aircraft designed around the dynamic drive and rotor systems of the Sikorsky S-61, it was designed to serve as an attack helicopter or to transport up to eight troops into combat.
Document 13: Penetrator (aircraft): A penetrator is a term referring to long-range bomber aircraft designed to penetrate enemy defenses. The term is mostly applied to aircraft that fly at low altitude in order to avoid radar, a strategic counterpart to the shorter-ranged tactical interdictor designs like the TSR-2 and F-111. However, the term can be applied to any aircraft that is designed to survive over enemy airspace, and has also been used for the penetration fighter designs that were designed to escort the bombers.
Document 14: Mil Mi-17: The Mil Mi-17 (NATO reporting name: Hip) is a Russian helicopter in production at two factories in Kazan and Ulan-Ude. It is known as the Mi-8M series in Russian service. It is a medium twin-turbine transport helicopter. There are also armed gunship versions.
Document 15: Bell UH-1Y Venom: The Bell UH-1Y Venom (also called Super Huey) is a twin-engine, medium-sized utility helicopter, built by Bell Helicopter under the H-1 upgrade program of the United States Marine Corps. One of the latest members of the numerous Huey family, the UH-1Y is also called "Yankee", based on the NATO phonetic alphabet pronunciation of its variant letter.
ground_truth: United States