During the 1980s, McDonnell Douglas studied an AH-64B, featuring an updated cockpit, new fire control system and other upgrades. In 1988, funding was approved for a multi-stage upgrade program to improve sensor and weapon systems. Technological advance led to the program's cancellation in favor of more ambitious changes. In August 1990, development of the AH-64D Apache Longbow was approved by the Defense Acquisition Board. The first AH-64D prototype flew on 15 April 1992. Prototype testing ended in April 1995. During testing, six AH-64D helicopters were pitted against a bigger group of AH-64As. The results demonstrated the AH-64D to have a sevenfold increase in survivability and fourfold increase in lethality compared to the AH-64A. On 13 October 1995, full-scale production was approved; a $1.9-billion five-year contract was signed in August 1996 to upgrade 232 AH-64As into AH-64Ds. On 17 March 1997, the first production AH-64D flew. It was delivered on 31 March.