The Republic F-105 Thunderchief is an American fighter-bomber that served with the United States Air Force from 1958 to 1984. Capable of Mach 2, it conducted the majority of strike bombing missions during the early years of the Vietnam War. It was originally designed as a single-seat, nuclear-attack aircraft; a two-seat Wild Weasel version was later developed for the specialized suppression of enemy air defenses (SEAD) role against surface-to-air missile sites. The F-105 was commonly known as the "Thud" by its crews. It is the only American aircraft to have been removed from combat due to high loss rates.

As a follow-on to the Mach 1 capable North American F-100 Super Sabre, the F-105 was also armed with missiles and a rotary cannon; however, its design was tailored to high-speed low-altitude penetration carrying a single nuclear weapon internally. First flown in 1955, the Thunderchief entered service in 1958. The single-engine F-105 could deliver a bomb load greater than some American heavy bombers of World War II such as the Boeing B-17 Flying Fortress and Consolidated B-24 Liberator. The F-105 was one of the primary attack aircraft of the Vietnam War, with over 20,000 Thunderchief sorties flown. Out of the 833 produced, 382 aircraft were lost, including 62 operational (non-combat) losses. Although less agile than smaller MiG fighters, USAF F-105s were credited with 27.5 kills.

During the conflict, the single-seat F-105D was the primary aircraft delivering heavy bomb loads against the various military targets. Meanwhile, the two-seat F-105F and F-105G Wild Weasel variants became the first dedicated SEAD platforms, fighting against the Soviet-built S-75 Dvina (NATO reporting name: SA-2 Guideline) surface-to-air missiles. Two Wild Weasel pilots were awarded the Medal of Honor for attacking North Vietnamese surface-to-air missile sites, with one shooting down two MiG-17s the same day. The dangerous missions often required them to be the "first in, last out", suppressing enemy air defenses while strike aircraft accomplished their missions and then left the area.

When the Thunderchief entered service it was the largest single-seat, single-engine combat aircraft in history, weighing approximately 50,000 pounds (23,000 kg). It could exceed the speed of sound at sea level and reach Mach 2 at high altitude. The F-105 could carry up to 14,000 lb (6,400 kg) of bombs and missiles. The Thunderchief was later replaced as a strike aircraft over North Vietnam by both the McDonnell Douglas F-4 Phantom II and the swing-wing General Dynamics F-111 Aardvark. However, the "Wild Weasel" variants of the F-105 remained in service until early 1984, at which point they were replaced by the specialized F-4G "Wild Weasel V".

Republic Aviation started work on what would become the Thunderchief during 1951. It was conceived of as an internal project to produce a replacement for the RF-84F Thunderflash, which first used the characteristic wing-root air intakes to make room for cameras in the nose section. The design team led by Alexander Kartveli examined some 108 configurations before settling on a large, single-engine AP-63FBX (Advanced Project 63 Fighter Bomber, Experimental), specifically AP-63-31. The new aircraft was intended primarily for supersonic, low altitude penetration to deliver a single, internally carried nuclear bomb. The emphasis was placed on low-altitude speed and flight characteristics, range, and payload. The aircraft would be fitted with a large engine, and a relatively small wing with a high wing loading for a stable ride at low altitudes, and less drag at supersonic speeds. Traditional fighter attributes such as maneuverability were a secondary consideration.

During April 1952, Republic made its contractor proposal, which contained many of the features that the United States Air Force (USAF) would have liked the RF-84F to have been outfitted with had it been technically possible; one month later, the Air Staff endorsed its development over that of further developing the RF-84F. The USAF promptly issued Republic with an initial contract covering preproduction engineering, tooling, as well as the production of 199 aircraft, the first of which was to be operationally ready by 1955. However, by March 1953, the USAF had reduced the order to 37 fighter-bombers and nine tactical reconnaissance aircraft, citing the approaching end of the Korean War. In October 1953, the F-105 mock-up was inspected; no major changes were recommended. By this point, the aircraft had grown so large that the Allison J71 turbojet intended for it was abandoned in favor of the more powerful Pratt & Whitney J75. Anticipating a protracted development of the engine, it was expected that the first aircraft would use the smaller Pratt & Whitney J57. Near the end of 1953, the entire program was suspended by the USAF due to a number of delays and uncertainties regarding the aircraft. However, on 28 June 1954, the USAF officially ordered 15 F-105s (two YF-105As, four YF-105Bs, six F-105Bs, and three RF-105Bs) under the Weapon System designation WS-306A.

On 22 October 1955, the YF-105A prototype made its maiden flight. After 22 hours of flight time, the prototype was returned to the factory for repairs after sustaining major damage. The second YF-105A made its first flight on 28 January 1956. Despite being powered by a less potent J57-P-25 engine with 15,000 pounds-force (67 kN) of afterburning thrust, the first prototype attained the speed of Mach 1.2 on its maiden flight. (The J75 was expected to generate 24,500 lbf (109 kN) with afterburner.) Both aircraft featured conventional wing root air intakes and slab-sided fuselages typical of the early jets; Republic viewed the prototypes as not being representative of the true capability of the aircraft due to numerous changes that were enacted prior to production. Specifically, insufficient power and aerodynamic problems with transonic drag, as well as Convair's experience with their F-102, had led to a redesign of the fuselage to conform to the area rule, giving it a characteristic "wasp waist". The F-105's reworked air intake was designed with a unique forward-swept shape, which was derived from Antonio Ferri's work on the proposed Republic XF-103 ramjet-powered interceptor. In combination with the distinctive forward-swept variable-geometry air intakes, which regulated airflow to the engine at supersonic speeds, and the J75 engine, this redesign enabled the F-105B to attain Mach 2.15.

During March 1956, the USAF placed a further order for 65 F-105Bs and 17 RF-105Bs. In order to conduct the nuclear mission, an MA-8 fire control system, AN/APG-31 ranging radar, and K-19 gunsight to allow for toss bombing were integrated. On 26 May 1956, the first pre-production YF-105B made its maiden flight. Five of the F-105C trainer variant were added to the procurement plan in June 1956, before being canceled during 1957. The RF-105 reconnaissance variant was canceled in July 1956. The first production F-105B was accepted by the USAF on 27 May 1957. In June 1957, Republic Aviation requested that the F-105 be named Thunderchief, continuing the sequence of the company's Thunder-named aircraft: P-47 Thunderbolt, F-84 Thunderjet, and F-84F Thunderstreak. The USAF made this name official one month later.