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. Weapons and sensor systems were integrated and tested during this time, including the laser-guided AGM-114 Hellfire missile. 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), for the purpose of arming helicopter platforms with an effective anti-tank missile.