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-114L 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  ωɛλρσɳ. 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, ωɛλρσɳs and navigation systems. The forward avionics bay is expanded, and the landing gear fairings are extended forward to accommodate some of the new equipment.

This gunship can carry a total of 16 AGM-114L Hellfire 2 anti-tank missiles. It can also carry 2 AGM-122 Sidearm air-to-ground anti-radiation missiles, that can be used to engage hostile air defense systems. For self-defense against helicopters the Longbow Apache can carry 4 AIM-92 Stinger, or 4 Mistral, or 2 AIM-9 Sidewinder short-range air-to-air missiles.

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 flown by Israel, the Netherlands and the United Kingdom (where it was built under license for the Royal Air Force by Westland as the WAH-64D).

The AH-64E Apache Guardian is the latest version of the Apache. Until 2012 it was designated as AH-64D Block III. It has a number of improvements and upgrades, including more powerful engines, upgraded transmission and other improvements. This helicopter might be also fitted with updated Longbow fire control radar. US armed forces plan to upgrade a total of 634 AH-64D helicopters to AH-64E standard. Deliveries began in 2011. This helicopter has been approved for export. Export operators are Saudi Arabia and Taiwan. This helicopter has been ordered by some other countries. India ordered 22 of these gunships, Indonesia 8, Iraq 24, and South Korea 36.