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

Context paragraphs:
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.

Aérospatiale SA 321 Super Frelon: The Aérospatiale (formerly Sud Aviation) SA 321 Super Frelon ("Super Hornet") is a three-engined heavy transport helicopter produced by aerospace manufacturer Sud Aviation (later Aérospatiale) of France.  It held the distinction of being the most powerful helicopter to be built in Europe at one point, as well as being the world's fastest helicopter.

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.

Kawasaki KH-4: The Kawasaki KH-4 was a light utility helicopter produced in Japan in the 1960s as a development of the Bell 47 that Kawasaki had been building under licence since 1952.  The most visible difference between the KH-4 and its forerunner was its new and enlarged cabin.  This was fully enclosed (although the side doors were removable) and provided seating for three passengers side-by-side on a bench seat behind the pilot's seat.  The helicopter was provided with a new control system, revised instrumentation, and larger fuel tank.

Agusta-Bell AB.102: The Agusta AB.102 was an Italian helicopter produced in small numbers in the early 1960s.  The aircraft was based on the mechanical components of a Bell 48 that Agusta incorporated into an all-new, streamlined fuselage.  The first flight was on 3 February 1959 and the prototype was exhibited at that year's Paris Air Show in "faux" military colours.  Only two production examples were built, operated by Elivie in a regular air service between Turin and Milan from 1961.  However, the advent of turbine-powered helicopters in the 1960s soon rendered the AB.102 obsolete.

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.

Eurocopter EC135: The Eurocopter EC135 (now Airbus Helicopters H135) is a twin-engine civil light utility helicopter produced by Airbus Helicopters (formerly known as Eurocopter).  It is capable of flight under instrument flight rules (IFR) and is outfitted with digital flight controls.  It entered service in 1996; over a thousand aircraft have been produced to date.  It is widely used by police and ambulance services and for executive transport; by 2013, more than 500 EC135s were providing helicopter emergency medical services across the world.  While the EC135 is primarily used by civil operators, a combat-capable military-orientated variant of the EC135, designated as the Eurocopter EC635, has also been produced.

Robinson R44: The Robinson R44 is a four-seat light helicopter produced by Robinson Helicopter Company since 1992.  Based on the company's two-seat Robinson R22, the R44 features hydraulically assisted flight controls.  It was first flown on 31 March 1990 and received FAA certification in December 1992, with the first delivery in February 1993.

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.

Brantly B-2: The Brantly B-2 is an American two-seat light helicopter produced by the Brantly Helicopter Corporation.

Supporting paragraphs: American Aircraft Penetrator Bell UH-1 Iroquois
Answer: United States