It has a NATO ranking code of OF-1 and is equivalent to a second lieutenant in the British Army or the Royal Marines. The Royal Navy has no exactly equivalent rank, and a pilot officer is senior to a Royal Navy midshipman and junior to a Royal Navy sub-lieutenant. In the Australian Armed Forces, the rank of pilot officer is equivalent to acting sub lieutenant in the Royal Australian Navy.
The equivalent rank in the Women's Auxiliary Air Force (WAAF) was "assistant section officer".
In the Royal Flying Corps, officers were designated pilot officers at the end of pilot training. As they retained their commissions in their customary ranks (usually second lieutenant or lieutenant), and many of them had been seconded from their ground units, the designation of pilot officer was a position title rather than a rank.
On 1 April 1918, the newly created RAF adopted its officer rank titles from the British Army, with Royal Flying Corps second lieutenants becoming second lieutenants in the RAF. Consideration was given to renaming second lieutenants as ensigns. However, when the RAF's own rank structure was introduced on 1 August 1919, RAF second lieutenants who were qualified pilots were redesignated pilot officers, a rank which has been in continuous use ever since. Those who were not qualified pilots were redesignated observer officers, but this was later phased out and all officers of this rank became pilot officers.
The rank of pilot officer does not imply that the officer is aircrew. Following recent reforms to the Royal Air Force's promotion system, wherein previously University graduates passed out of RAF Cranwell at a higher substantive rank than their non-graduate peers, pilot officer rank is now only applicable to ground branches. Aircrew and engineers receive their commissions as flying officers and skip the rank altogether. A ground branch officer will remain in the pilot officer rank for six months following commissioning, before an automatic promotion to flying officer. Because of the nature of Phase II training (professional training after the Phase I Initial Officer Training), a pilot officer will generally spend time in rank on a further training course, and is not likely to be operationally active.