It has a NATO ranking code of OF-2, and is equivalent to a lieutenant in the Royal Navy and a captain in the British Army and the Royal Marines. The equivalent rank in the Women's Auxiliary Air Force (WAAF), Women's Royal Air Force (WRAF) (until 1968) and Princess Mary's Royal Air Force Nursing Service (PMRAFNS) (until 1980) was flight officer.
On 1 April 1918, the newly created RAF adopted its officer rank titles from the British Army, with Royal Naval Air Service lieutenants (entitled flight lieutenants and flight commanders) and Royal Flying Corps captains becoming captains in the RAF. In response to the proposal that the RAF should use its own rank titles, it was suggested that the RAF might use the Royal Navy's officer ranks, with the word "air" inserted before the naval rank title. For example, the current rank of flight lieutenant would have been "air lieutenant". Although the Admiralty objected to this simple modification of their rank titles, it was agreed that the RAF might base many of its officer rank titles on navy officer ranks with differing pre-modifying terms. It was also suggested that RAF captains might be entitled flight-leaders. However, the rank title flight lieutenant was chosen as flights were typically commanded by RAF captains and the term flight lieutenant had been used in the Royal Naval Air Service. The rank of flight lieutenant has been used continuously since 1 August 1919.
Although in the early years of the RAF a flight lieutenant commanded an aircraft flight, with the increasing combat power of aircraft and therefore squadrons, command and control has shifted up the rank structure (currently, for instance, most squadron commanders in the RAF are Wing Commanders, a reflection on the comparative combat power between the modern air force and its precedent).
The RAF's promotion system is automatic up until Flight Lieutenant. Every officer will attain the rank provided they complete their professional training and do not leave early. For Aircrew, Flight Lieutenant is reached 2.5 years after commissioning, BEng/MEng qualified engineers 2.5 and 1.5 years respectively, and for all ground branch officers, 3.5 years. Aircrew are appointed to an Early Departure Payment Commission upon reaching their Operational Conversion Unit, which is a commission for 20 years or age 40, whichever is later. Promotion to Squadron Leader thereafter is strictly upon merit; officers promoted beyond Flight Lieutenant are appointed to a Career Commission, or service to age 60. Resigning a commission is generally dependent on the needs of the Service, although an officer who has completed their Return of Service (service the RAF requires to justify its expense in originally training the officer) could leave after as little as four years. For aircrew, given the large expense required for training, this Return of Service is generally the length of their initial commission anyway, unless they re-role to a different branch having failed an element of flying training. Most aircrew reach their squadrons as Flight Lieutenants due to the length of training time required (up to four years for fast jet pilots). The majority of squadron line pilots are flight lieutenants, with some squadron executives or Career Commission aircrew reaching Squadron Leader.