Beginning in the late 1980s, the US Army planned a series of upgrades to its AH-64A fleet. The major upgrade is centred around the Northrop Grumman APG-78 Longbow milimetric-wavelength fire-control radar allied to new AGM-114L Hellfire 2 missiles. During 1992 McDonnell Douglas converted four AH-64As with this radar to act as proof-of-concept aircraft for a variant designated AH-64D. The Designations AH-64B and AH-64C for interim variants were later dropped so that the AH-64D Apache became the second operational Apache variant.
Longbow is readily identifiable by the mast-mounted antenna for its radar. It allows the AGM-114 L to be fired in an autonomous fire-and-forget mode, whereas the laser-guided Hellfire requires external designation or use in conjunction with the TADS, and as such is a line-of-sight and non fire-and-forget weapon. The APG-78 radar can detect, classify and prioritise 12 targets simultaneously, and can see through the fog an smoke that currently foils infra-red or TV sensors.
The AH-64D also features improvements in targeting, battle management, cockpit, communications, weapons and navigation systems. The forward avionics bay is expanded, and the landing gear fairings are extended forward to accommodate some of the new equipment.
Entering service in 1995, early aircraft lack the radar system fitted to the definitive AH-64D Longbow Apache that followed from 1997. Early in 1999 the US Army finally decided that 530 AH-64As would be upgraded to D standard, for which 500 Longbow systems would be procured, and that the other 218 surviving AH-64As would be passed to the Air National Guard as a partial replacement for its Bell AH-1s. The AH-64D is also be flown by Israel, the Netherlands and the United Kingdom (where it is built under license for the RAF by Westland as the WAH-64D).