For nearly three decades, the F-15 Eagle fighter was considered the undisputed king of the skies. Until the debut of its replacement, the F-22 Raptor, the F-15 was the US Air Force’s frontline air superiority fighter. Even today, a modernized Eagle is still considered a formidable opponent, and manufacturer Boeing has proposed updated versions that could keep the airframe flying for the better part of a century.

According to a special report by Kyle Mizokami, in The National Interest, the F-15 traces its roots to the air war in Vietnam, and the inauspicious showing of American Air Force and Navy fighters versus their North Korean counterparts.

Large, powerful American fighters, designed to tackle both air-to-air and air-to-ground missions, were performing poorly against their smaller, less powerful — but more maneuverable— North Vietnamese counterparts. The 13:1 kill ratio American fliers enjoyed in the Korean War dropped to an abysmal 1.5 to 1 kill ratio in Vietnam.

Contemporary fighters, such as the F-4 Phantom, had been designed under the assumption that the air-to-air missile had rendered dogfights obsolete, and with them the need for superiority maneuverability and a gun for air combat.

The USAF decided it needed a dedicated air superiority fighter, one that combined two powerful engines, a powerful radar, a large number of missiles and a gun. Above all, it had to be maneuverable enough to win a dogfight.

The Air Force issued a request for proposals for the new FX fighter in 1966, and no fewer than six companies submitted competing paper designs. No prototypes were built. The air service selected McDonnell Douglas (now a part of Boeing) in 1969, ordering 107 full-scale development planes.

The F-15 was a formidable aircraft. Early versions were powered by two Pratt & Whitney F100-PW-100 after-burning turbofan engines, producing 14,500 pounds of static thrust—23,500 with afterburners.

This gave the aircraft a thrust-to-weight ratio of greater than one, making it so powerful that it was the first fighter to exceed the speed of sound in vertical flight. The F-15 had so much thrust it could climb to sixty-five thousand feet in just 122 seconds. In horizontal flight, the F-15 could reach speeds of Mach 2.5, and cruise at speeds of Mach 0.9.

The Eagle’s AN/APG-63 nose mounted radar was the most advanced of its day, a solid state radar with “look down/shoot down” capability and a range of up to 200 miles. This allowed the F-15 to pick out low-flying enemies on radar against the clutter generated by the ground.

The radar was also the first to incorporate a programmable system processor, which allowed moderate upgrades to be done via software and not intrusive hardware updates.

The Eagle was originally armed with four radar-guided AIM-7 Sparrow missiles for long-range engagements and four AIM-9 Sidewinder infrared guided missiles for short-range engagements. In the air war over Vietnam, USAF F-4C Phantoms, lacking a dedicated gun, missed several opportunities to down enemy aircraft. This was remedied in the F-15 by equipping the plane with an internal M61 Vulcan 20mm gatling gun.

The F-15 was also designed with long range in mind. Carrying three six-hundred-pound fuel tanks, the F-15 had a range of three thousand miles, making it possible to fly from the continental United States to Europe without stopping or midair refueling. This would make it possible to quickly reinforce NATO air defenses in case of a crisis in Europe, and later would permit the Air Force to quickly dispatch F-15s to Saudi Arabia during Operation Desert Storm.

The first F-15 prototypes flew in 1972, and serial production began in 1973. The plane rapidly began to populate both the USAF and friendly air forces, including Israel, Japan and Saudi Arabia. The F-15’s first air-to-air kill was on June 27 1979, when Israeli Air Force ace Moshe Melnik shot down a Syrian Air Force MiG-21 in his F-15A. Melnik would eventually down four aircraft from F-15As and F-15Cs, for a career total of eleven enemy fighters shot down.

Melnik’s kill was the start of a remarkable string of 104 consecutive air-to-air victories for the F-15, with not a single Eagle lost. Israeli, Saudi and American F-15s were responsible for this impressive streak. Israeli kills included were recorded between 1979 and 1982 and included Syrian MiG-25 Foxbat interceptors, MiG-21 and MiG-23 fighters, and a number of ground attack and strike aircraft.

During the 1991 Gulf War, the American and Saudi tally included Iraqi MiG-29 Fulcrum fighters, Mirage F-1 fighters and even an Il-76 “Candid” medium transport. One F-15E Strike Eagle even scored an air-to-air kill against an Iraqi Mi-24 attack helicopter with a laser-guided bomb.

The F-15A was eventually replaced in production by the F-15C, which included a newer AN/APG-70 synthetic aperture radar and newer F100-PW-220 engines. The latest program, nicknamed Golden Eagle, stress tested F-15Cs for wear and tear, and 178 of the planes in the best physical condition received new APG-63V3 active electronically scanned array radars and the Joint Helmet Mounted Cuing System, allowing rapid target acquisition.

In the late 1980s, the F-15E was developed to supplement — and eventually replace the F-111 fighter bomber as a penetrating, high speed tactical strike aircraft designed to strike deep behind enemy lines in a NATO/Warsaw Pact war in Europe.

The E model added conformal fuel tanks to increase range with a heavy bomb payload, the APG-63 radar, and a LANTRIN forward-looking infrared and laser targeting pod. With the retirement of the F-111, the F-15E “Strike Eagle” is now the USAF’s main tactical fighter bomber.

Meanwhile, in February of this year, the USAF announced it will finally go ahead and buy brand-new F-15s even as it purchases large numbers of the F-35 Joint Strike Fighter. Updated with the latest technology, the F-15X can carry nearly two dozen air-to-air missiles and will likely work together with stealth jets to take on fleets of enemy fighters, Popular Mechanics reported.

Media reports revealed that the USAF will request eight Advanced Eagle F-15X fighters in its budget, with plans to buy 80 fighters over five years, and possibly more after that. Enough to fit out a wing of 72 aircraft, divided into three squadrons of 24 planes each, with eight spares.

The F-15X will also include large flat-panel displays for displaying aircraft information, conformal fuel tanks to give it a longer range, a digital fly-by-wire control system, a new APG-82 radar, and the Eagle Passive Active Warning Survivability System (EPAWSS) for protection from air-to-air missiles.

Congress has yet to pass a defense appropriations bill for fiscal year 2020, and the Pentagon and other federal agencies have been operating under a continuing resolution since Oct. 1.

Why does the Air Force want to buy a better version of an older non-stealth fighter when it’s already got plenty of F-35s on the way? The running theory is that the F-15X will complement the F-22 Raptor and the F-35 Joint Strike Fighter in the air combat mission.

Today, the USAF still employs around 177 upgraded F-15C and two-seater D models, and approximately 224 F-15E Strike Eagles. F-15s are deployed in forward bases in both Europe and Asia, most notably at RAF Lakenheath in the UK and Kadena Air Force base on the Japanese island of Okinawa.