F-4E
Combat experience led to the definitive Air Force "F-4E", which performed its initial flight on 30 June 1967. The F-4E featured:

Uprated J79-GE-17 engines, with 79.6 kN (8,120 kgp / 17,900 lbf) afterburning thrust.

A General Electric M61A1 six-barreled 20-millimeter Gatling-type cannon on the underside of the nose, with 640 rounds of ammunition. Fit of the cannon required a new and longer nose, a modified version of the nose designed for the RF-4C reconnaissance Phantom, described below. The cannon pod had been a big help, but it was "draggy" and tended to become misaligned on its stores pylon, making it inaccurate. The built-in cannon was a much better solution. However, early production F-4Es suffered from engine "flameouts" due to ingestion of gases expelled from the muzzle of the cannon, and to fix this problem GE quickly came up with a longer-barreled cannon, which was fitted into a longer cannon fairing with improved ventilation. This fix was retrofitted to earlier F-4E production.

A solid-state AN/APQ-120 radar. The new radar was much more compact than its predecessors, allowing it to fit into the nose along with the cannon.

Martin-Baker Mark 7 "zero-zero (zero altitude, zero speed)" ejection seats.

A tailplane with a fixed slat attached under the leading edge. The slat provided improved control authority at high angles of attack (AOA).

A seventh fuel cell in the tail above the engine nozzles.

Manual wing fold, instead of hydraulic fold. The Air Force usually didn't need to fold the wings unless Phantoms were being transported, and so the hydraulic fold was effectively dead weight.

The F-4E became the primary USAF variant, and was also heavily exported, as is discussed later. Including exports, 1,387 were built into 1979.

The F-4E was enhanced during and after its production. One of the particular limitations of the Phantom was that it had been designed basically as an interceptor and strike aircraft, not a dogfighter, and maneuverability had suffered as a result. The lack of maneuverability proved troublesome in combat -- in the worst case, under certain aggressive flight conditions the F-4 could go into a spin that generally required immediate ejection if the aircraft was below 3,000 meters (10,000 feet). This unpleasant behavior came as something of a nasty shock, since flying the Phantom under peacetime conditions was very straightforward, one pilot calling it "idiot-proof", and it had an admirable flight safety record.

In response to the maneuverability problems, a program named AGILE EAGLE was initiated, in which the Phantom was fitted with various experimental aerodynamic modifications. The solution in the end was to fit the wing with leading-edge slats, which extended automatically when the aircraft's AOA exceeded six degrees, and retracted when the AOA went back to four degrees. Installation of the slats required removal of the BLC system. The leading-edge slats were introduced into F-4Es beginning in mid-1972 and were retrofitted to older aircraft. Beginning the next year, 1973, F-4Es went through a structural strengthening program after an F-4E was lost due to a wing structural failure.

The Northrop "Target Identification System Electro-Optical (TISEO)" camera system was also introduced in late F-4E production, being retrofitted to older aircraft as well. TISEO involved a steerable, stabilized telescopic camera mounted on the left wing, and was used primarily to inspect targets before engaging them. TISEO imagery was displayed on the WSO's radarscope. The camera could be slaved to the AN/APQ-120 radar, and apparently there was an upgrade that allowed it to be linked to the navigation system to target flight checkpoints as well. Of course, it was a daylight / clear-weather only system. Most export F-4Es were not fitted with TISEO.