In RAF service, the Folland Gnat was a popular and diminutive trainer; in the air forces of Finland and India, however, the Gnat was a suprisingly capable lightweight single-seat fighter whose performance was completely out of proportion to its size. Designed by W. E. Petter (the man behind such classic aircraft like the Canberra and Lightning), the Gnat had been designed as an export oriented fighter, to be built in countries whose aircraft industries were still in their infancy. One such country was India, who recieved both Folland and HAL-built units and used them very successfully during the Indo-Pakistani wars to the point that it was given the flattering nickname 'Sabre slayer' on account on the ease with which it could take down the famed dogfighter which was the main fighter used by Pakistani forces. With the RAF, the Gnat was used mostly as a trainer and with the famous Red Arrows acrobatic display unit until being replaced in both roles by the BAe Hawk.

The appropriately named Fo.139 Midge was the immediate precursor to the Fo.144 whose private venture prototype had its maiden flight on 18 July 1955. Single seat units were known as the Gnat F.1 but not used by the RAF which instead exported them to Finland and India while retaining the T.1 trainer until 1979. Indian units were both British and locally built by Hindustan Aeronautics (HAL) and led to the improved Ajeet (Unconquerable).