Hands down one of the most revolutionary designs in aviation history, the Hawker Siddeley Harrier was the world's first Vertical/Short Takeoff or Landing (V/STOL) combat aircraft to enter service. It's origins lay in the 1957 Hawker P.1127 prototype which used engines with movable nozzles to provide the necessary vectored thrust for taking off and landing vertically. It entered service the following decade with the RAF, which valued its ability to easily operate without the need of an airfield, given the threat of Soviet conventional and attacks on NATO bases in Germany the event of war. Around the same time, the US Marine Corps also saw its potential for use on its amphibious assault carriers. Subsequent collaboration with the US would also result in a second-generation Harrier known as the Harrier II. Featuring a significantly redesigned airframe, the Harrier II had greatly improved combat capabilities including heavier payloads and (later) precision bombing. In RAF service, first generation Harriers have served with distinction in the Falklands War where they flew from Royal Navy carriers initially, while second generation Harriers have participated in the campaigns over Bosnia, Kosovo, Iraq, and Afghanistan (RAF Harriers were not employed in the 1991 Gulf War although their USMC counterparts did see action). Other foreign users besides the US include Spain, India, and Thailand; additionally Italy has also adopted US variants. In the 2000s The RAF's Harrier fleet merged with that of the FAA into the Joint Force Harrier (later Joint Strike Wing) but it was eventually disbanded in 2010 as a result of post-crisis budget cuts.

First flight of the P.1127 prototype was on 21 October 1960 although true horizontal flight was not achieved until one year later. Initial production versions were designated Harrier GR.1 and entered service as early as 1969. These were subsequently upgraded first into the GR.1A (more powerful engine) and then the GR.3 with a lengthened nose housing a laser target seeker. Further versions of the aircraft would be joint ventures between British Aerospace and McDonnell Douglas and known as the Harrier II, these featured larger wingspans which allowed for heavier payloads. A raised cockpit also provided superior visibility. The first production variant was the GR.5 which differed from the US AV-8B by having a different nose and two extra pylons, usually reserved for Sidewinder missiles. This was followed by the night-attack GR.7 with FLIR, a redesigned cockpit, and other advanced avionics but lacked the gun pods. Precision-bombing capability was added with the integration of Maverick missiles and targeting pods from the late 1990s leading to the final GR.9 which could carry more advanced Paveway IV bombs and SNIPER pods. Aircraft with uprated Mk. 107 engines are known as GR.7A and GR.9A. Two-seat trainer versions include the T.4, T.8 and T.10. Spanish first generation aircraft are known as the AV-8S Matador and TAV-8S (trainers), with some of these later transferred to Thailand. No second generation RAF variants have been exported.