Republic Aviation started the Thunderchief as an internal project to replace the F-84F Thunderflash. 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). The new aircraft was intended primarily for supersonic, low altitude penetration into the Soviet Union on a hi-lo-hi mission and delivery of a single, internally carried nuclear bomb. Thus, with emphasis 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 which would give a stable ride at low altitudes, and less drag at supersonic speeds.

Traditional fighter attributes such as maneuverability were a secondary consideration. Enthusiastic at first, the United States Air Force awarded Republic with a contract for 199 aircraft in September 1952. However, by March 1953 the USAF had reduced the order to 37 fighter-bombers and 9 tactical reconnaissance aircraft, citing the approaching end of the Korean War. By the time the F-105 mock-up had been completed in October 1953, the aircraft had grown so large that the Allison J71 turbojet intended for it was abandoned in favor of an even more powerful Pratt & Whitney J75. Anticipating protracted development of the engine, it was expected that the first aircraft would use the smaller Pratt & Whitney J57. On 28 June 1954, the USAF officially ordered 15 F-105As under the Weapon System designation WS-306.

The YF-105A prototype first flew on 22 October 1955, with the second YF-105A following on 28 January 1956. In spite of being powered by a less potent J57-P-25 engine with 15,000 pound-force (66.7 kN) of afterburning thrust (the J75 was expected to generate 24,500 pound-force (109.0 kN) with the afterburner), the first prototype attained the speed of Mach 1.2 on its maiden flight. Both prototypes featured conventional wing root air intakes and slab-sided fuselages typical of the early jets. However, insufficient power and aerodynamic problems with transonic drag, as well as Convair's experience with their F-102 Delta Dagger, led to a redesign of the fuselage in order to conform to the Area rule, giving it a characteristic "wasp waist". 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 enabled the resulting F-105B to attain Mach 2.15.

Republic YF-105A (S/N 54-0098), the first of two prototypes). (U.S. Air Force photo)

The F-105 was a mid-wing monoplane with a 45 degree swept wing and tail surfaces. The single engine was fed by two intakes in the wing roots, leaving the nose free for a radome housing the multi-mode radar. At the time, the F-105 was the largest single-seat combat aircraft ever built. Its capacious fuselage provided room for 1,160 US gallons (4,460 L) of fuel and a bomb bay measuring 15 feet 10 inches by 32 inches by 32 inches (4.82 m x 0.81 m x 0.81 m), originally intended for a single nuclear weapon but typically containing an additional 390 US gallon (1,500 L) fuel tank. Two underwing and one fuselage wet-stores-capable pylons were provided for 450 and 650 US gallon (1,730 L and 2,500 L) expendable fuel tanks. Two outboard (dry) stations were wired for missiles or bombs. A single T-171E3 20 millimeter Gatling cannon was installed in the left side of the nose with a magazine for 1,028 rounds of ammunition, combined with a radar ranging gunsight in B and D models. The aircraft was designed to carry the short-range Sidewinder but it would not be equipped with the medium-range radar-guided missiles.

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