However, the all-missile fighter had shown some serious drawbacks in the initial air-to-air battles over Vietnam. The earlier Sparrow, Falcon, and Sidewinder air-to-air missiles did not perform up to expectations. They were expensive, unreliable, and vulnerable to countermeasures. Many an enemy MiG was able to escape unscathed because a Phantom-launched missile malfunctioned and missed its target. The Phantoms could carry a podded cannon mounted on the centerline, but it was relatively inaccurate, caused excessive drag which reduced the performance of the Phantom carrying it, and took up a valuable ordinance/fuel station.
An initial F-4 variant with an internal M61 cannon had been proposed by McDonnell to the USAF in March of 1961, but had met with little enthusiasm. McDonnell began a new design study for a gun-armed Phantom in late 1964 and finally got the attention of the Air Force. The gun-armed F-4E was finally funded in June of 1965. It was destined to be produced in greater numbers than any other single Phantom variant.
The main difficulty in equipping the Phantom with an internal cannon was in finding a place to put it. The solution was found in using the sharper, longer nose of the F-4C reconnaissance version. The new nose was fitted with an AN/APG-30 radar set and an external pod was mounted underneath the nose that could carry a single six-barrel 20-mm General Electric M61A1 rotary cannonThe severe space constraints in the new nose meant that a new ammunition feed system had to be designed for the M61A1 cannon. In addition, the proximity of the gun to the radar set required that very effective vibration dampers and noise/blast eliminators had to be designed.
An initial batch of 96 F-4Es was ordered in August 1966 as part of an F-4D contract. The first production F-4E (serial number 66-0284) flew on June 30, 1967, R. D. Hunt and Wayne Wight being the crewmembers.
The gun installation underneath the nose precluded the installation of the large radar set that was fitted to the F-4C and F-4D, so the F-4E carried the solid-state Westinghouse AN/APQ-120 X-band radar set which had a smaller antenna. However, due to the late delivery of the AN/APQ-120 radar, the first 30 F-4Es were delivered without any radar at all. Most were fitted with the new radar when it eventually became available.
The weight of the gun and its 639-round ammunition drum was counterbalanced by fitting an additional 95-gallon fuel tank in the rear fuselage, bringing total internal fuel capacity to 1993 gallons. One of the two fin-mounted pitots (the upper one) was relocated to the extreme nose. The F-4E retained the semi-recessed AIM-7 Sparrow air-to-air missiles and the external store stations of the earlier variants. The engines were a pair of J79-GE-17 engines with an afterburning thrust of 17,900 pounds. In the interest of eliminating excess weight, the powered folding wing mechanism of the earlier USAF Phantoms was finally eliminated. Also deleted was the emergency ram-air turbine what sat inside a recess on the upper rear fuselage.
The second production F-4E (66-0285) flew for the first time on September 11, 1967. It differed from 66-0285 by having a slotted stabilator. This slotted stabilator was added in order to give greater tailplane effectiveness, helping to counteract the increased weight in the nose. The second production F-4E also introduced the long "turkey feather" afterburner, which became a trademark of the F-4E. As the first fully aerodynamic representative F-4E, 66-0285 was earmarked for spin testing.
It turned out that the elimination of the emergency ram-air turbine was a mistake, and some sort of emergency power source was needed in case of engine failure. Consequently, starting with Block 40 (68-0452), an auxiliary power unit was added underneath the stabilator. However, it was only a battery-powered electrically-driven hydraulic pump and was not a small turbine engine. It provided just enough control to allow time for ejection, and probably would not last long enough to allow a landing.
Starting with Block 41 (68-0495 and beyond), the fuselage bladders were replaced by self-sealing fuel tanks. This reduced internal fuselage fuel capacity from 1364 to 1225 US gallons.
The most significant change at 71-0237 was the replacement of the blown leading-edge wing droops of earlier Phantoms by slats. This was done in the interest of obtaining enhanced combat maneuverability, which had been one of the Phantom's weak points. The outer leading edge slats were were driven by a hydraulic jack and terminated in a large "dogtooth" at the inboard end where the wing folding joint had once been. Immediately downstream of the dogtooth edge was a small wing fence. The inboard wing was also fitted with powered slats which terminated about three feet from the root. The inner 3 feet of the leading edge were fixed.
The first production F-4E to be fitted with slats was 71-0237, but the first to actually fly with slats was 71-0238 which made its maiden flight on February 11, 1972. The addition of these slats greatly enhanced the maneuvering performance, and the USAF decided to retrofit earlier F-4Es with these slats. The USAF ordered the first slat modification kits in April of 1972, and the first retrofitted F-4E (serial number 69-7524) flew on September 28, 1972. 304 earlier production block F-4Es were retrofitted with these slats, which included just about every surviving F-4E except for those serving with the Thunderbirds.
Block 53 also introduced the J79-GE-17Cor -17E with a low-smoke combustor. Earlier Phantoms had the annoying habit of leaving a trail of black smoke behind them, making them easier to spot by enemy gunners on the ground.
At about the same time, the gun installation underwent a major design. From the beginning, the sheer power of the muzzle blast and the highly-explosive gun gases generated during firing had created severe problems for the design team. With the original gun muzzle design, the F-4E often experienced engine flameouts caused by ingestion of gun gases into the engine intakes. In addition, the shape of the muzzle often produced a loud whistle which could be heard on the ground long before the approaching aircraft actually appeared. These problems were eventually cured by adding a long blast diffuser to each of the six barrels, joined to the barrel by a stripper diffuser which ejected most of the gun gas sideways and also decelerated and cooled the blast. A ram inlet was fitted above the forward fuselage to blast fresh air through the gun compartments. This inlet opened during gun firing and remained open for 30 seconds after the gun stopped firing. In addition, a "derichment system" was added which was triggered by the gun-firing circuit and enabled either engine to dump gas-enriched air overboard before it could enter the engine compressor and cause stalls or flameouts. These modifications came to be known as the "Midas 4". These modifications were introduced from Block 48 onward and were retrofitted to earlier blocks. Externally, the modified Midas 4 update could be recognized by a distinct projection protruding out in front of the gun compartment which extended forward underneath the radome.
The F-4E stayed in production for twelve years, and was built for more air forces and in larger numbers than any other Phantom variant. A total of 1387 F-4Es were built before production came to an end. 993 of these machines were intended for the USAF, with the remaining 394 being delivered new to foreign customers. 24 USAF F-4Es were taken from store and loaned to foreign customers, and 191 were passed on to foreign customers from USAF stocks. The last F-4E (an F-4E intended for Korea) left the production line at McDonnell on October 25, 1979. This brought domestic production of the Phantom to an end.
993 F-4Es were built for the USAF. Included in this total are 10 F-4E-63-MCs purchased by Germany for use in a joint US/German training program at George AFB in California, plus 58 "payback" F-4E-60-MC to 62-MCs acquired as replacements for aircraft that were hastily transferred by the USAF to Israel during the Yom Kippur War of October 1973.
The first F-4Es reached the Southeast Asia theatre in November of 1968, equipping the 469th TFS at Korat in Thailand. The 4th FTS and 421st TFS arrived from CONUS in April 1969 with F-4Es to join the F-4Ds of the 366th TFW at Da Nang AB. After this, the F-4Ds of the 366th TFW assumed forward air control duties, whereas the F-4Es conncentrated on aircraft escort duties and conducted ground attack missions. Six more F-4E squadrons deployed to Vietnam and Thailand in 1972 in response to the North Vietnamese invasion of the South in the spring of 1972.
The F-4E was credited with 21 MiG kills during the war. 10 of these were brought down by Sparrows, five with gunfire, four with Sidewinders, one with a combination of Sidewinder and gunfire, and one while maneuvering (no weapons being fired). However, most combat missions flown in Vietnam by the F-4E were ground-attack missions.
Beginning in 1975, 116 F-4E-42-MC through -45-MCs were converted to F-4G Wild Weasel defense suppression aircraft.
The F-4E began be supplanted in USAF frontline units by the newer F-15 Eagle starting in 1975 and by the F-16 starting in 1979. With the USAF in Europe, the last F-4Es were with the 52nd TFW at Spangdahlem in Germany which re-equipped with F-16s in 1988. The last two F-4E squadrons in the Pacific theatre were converted to F-16C/Ds in 1989. The TAC kept its F-4Es a bit longer, not relinquishing its machines until the early 1990s.
By the time of Desert Storm in January 1991, almost all of the F-4Es had been withdrawn from active duty USAF units, having been passed along to foreign customers or placed in storage. Nevertheless, a handful of Pave Tack capable F-4Es flew with the 7440th Composite Wing based at Incirlik AFB in Turkey, operating against targets in northwestern Iraq.
Two Air Force Reserve squadrons received F-4Es. These were the 457th TFS of the 201th TFW, which received F-4Es in 1987, and the 704th TFS of the 924th TFG, receiving F-4Es in 1989. Both of these squadrons traded in their F-4Es for F-16A/B fighters in 1991.