The highly successful Northrop F-5 is largely the result of private initiative. In 1955, Northrop undertook development of its N-156F fighter for the export market. The purpose was to achieve a simple, inexpensive, easily maintained multi-role fighter for sale to less-developed countries, but the project also led to the T-38 Talon trainer. With Lew Nelson as pilot, the US-financed N-156F flew on 30 July 1959 and the type later became known as the F-5A Freedom Fighter. The F-5B two-seat combat trainer actually became operational first, joining the 4441st Combat Crew Training Squadron at Williams AFB, Arizona, on 30 April 1964. By late 1964, both were being evaluated by the USAF, primarily to determine suitability for export to South Vietnam. The ultimate such test was Operation Skoshi Tiger, an extended deployment of USAF F-5As to South Vietnam over 1965-67. American pilots found the F-5A well-suited for the ground-attack mission and were bitter that, after these machines were turned over to the South Vietnamese in 1967, their own air force had none.

The F-5A and F-5B were exported to a dozen countries and were manufactured in Canada (CF-5A and CF-5D), the Netherlands (NF-5A and NF-5B) and Spain (SF-5A, SRF-5A and SF-5B). The RF-5A reconnaissance variant with four nose cameras was also widely used abroad, and the improved F-5C went to Vietnam during the empty years of the late 1960s when the US Air Force operated no aircraft of the type.

For a 1970 USAF-sponsored international fighter (IFX) competition, Northrop designed an improved version powered by two 5,000-lb (22.24-kN) thrust General Electric J85-GE-21 turbojets providing 23 per cent more power than the engines of the F-5A. Chosen over Lockheed, McDonnell and Vought submissions, this was initially called the F-5A-21 but became the F-5E Tiger II. The F-5E had increased fuel capacity and an integrated fire-control system (though without radar) but it retained the simplicity and ease of operation which made it a prime candidate for Third World countries. The first production F-5E was flown on 11 August 1972 with Hank Chouteau at the controls and was soon followed by the F-5F two-seat combat trainer with lengthened fuselage, which retained the fire-control system and combat capability. F-5Es were assembled under licence in Taiwan. Only long after foreign countries had purchased them did the USAF and US Navy acquire the F-5E and F-5F for aggressor squadrons to simulate MiG threat aircraft in dissimilar air combat training (DACT). In USAF service the type served with the 64th and 65th Aggressor Squadrons at Nellis AFB, Nevada, the 3rd AS at Clark AB, Philippines, and the 527th AS at RAF Alconbury, England. The 405th TTW at Luke AFB, Arizona, also operated the type on fighter training duties.

The F-5G designation was assigned to a single-engined variant originally intended for Taiwan, although sales to that customer became impossible after the US normalized relations with China in December 1978. The F-5G first flew at Edwards AFB on 30 August 1982 piloted by Russ Scott. By November, this variant had been renamed the F-20A Tigershark.

Some 818 F-5As (including the almost identical F-5C) and 290 F-5Bs were built.