In the period in question, the Fleet Air Arm operated two fighter aircraft with a rear gunner. These were the Blackburn Skua and Roc. The former was a fighter/dive-bomber, while the latter was a turret fighter like the RAF's Defiant. The tail guns in these aircraft could be manned either by an observer or a Telegraphist Air Gunner (TAG). Observers were generally naval officers, while the TAGs were always ratings. Their roles and training differed significantly from each other, and from the pilot training.

Observers had been introduced owing to the situation of the Fleet Air Arm in the late 1910s and early 1920s. Until 1924, it was entirely controlled by the RAF, which provided the aircraft, pilots and aircrew to operate from naval vessels. However, it soon became clear that the RAF's pilots didn't have the necessary experience in naval matters to, say, tell the difference between a fishing boat and a battleship. The Admiralty insisted on putting naval officers into aircraft to solve this problem, but were not allowed to train them as pilots. The observer was the resulting compromise, with the first courses opening in 1921. These early courses had three parts, beyond the standard training of a naval officer. The first part was a radio course at the Signal School in Portsmouth. This was followed by a two-week course at the gunnery school at HMS Excellent (on Portsmouth's Whale Island), to learn to spot the fall of shot for naval gunnery. The final phase was a course at the RAF base at Lee-on-Solent, learning navigation and putting their skills into practice in the air. From 1924, naval officers were allowed to become pilots in the Fleet Air Arm, though the observers would retain their importance, being generally senior officers. From 1939, the Fleet Air Arm was transitioned completely to the Navy, letting the RN set up training schools ashore. The Observer School moved to the newly established RNAS Ford. A newly enlisted observer would start with a period of training at HMS St Vincent, as would new pilots. Here, they would have an intense course in naval etiquette, as well as lessons on navigation, communications and meteorology. After this, they would be commissioned as RN officers. Pilots would go for flight training at the RAF bases at Elmdon and Luton, while observers went to RNAS Ford. Here, they would receive further training in navigation and communications, as well as in ship recognition and aerial photography. This training was both theoretical and practical, with observers taken aloft on multiple sorties, mainly to practice their navigation. Air gunnery training was also carried out here, with the observers training to fire against sleeve targets towed by aircraft. Following this training, they would be assigned to a Naval Air Squadron (though until 1939, they were assigned to a ship rather than a squadron), where they would be paired up with a pilot. From 1938, to bulk up the numbers of observers before the FAA's transition to the NR, ratings (enlisted men) were allowed to become observers, going through similar training.

TAGs, meanwhile, were always naval ratings. They could either be volunteer telegraphists from the rest of the RN, or new entries. New entries went through basic training at HMS Royal Arthur in Skegness, before moving onto St Vincent for further training. Then they went on to the TAG school. I don't have any information on this before 1939, as my books focus mainly on officer training. From 1939, though, the TAG school was based at RNAS Worthy Down. Here, they took a six-seven month training course. This was mostly ground-based, with theoretical and practical courses in radio communication and gunnery, their two main tasks. There were airborne elements to the course. 756 Squadron provided this for the radio course, with the TAGs taught how to home on beacons and the like in the air. 774 Squadron, based at RNAS St Merryn, meanwhile, taught air gunnery, using towed sleeve targets like those used for observer training. Once the training had finished, they were sent to the fighting squadrons.

Typically, FAA squadrons would assign crews on a near-permanent basis. This allowed crews to gain faimiliarity with each other, and to learn to best cooperate in the air. Aircrews were paired up when they joined the squadron, and would fly together after this. For Skuas and Rocs, which were two-seater aircraft, the squadrons would have a mix of TAGs and observers. Typically, the squadrons were divided into elements of three. The leader of each element would have an observer in his back seat, while the other two aircraft had TAGs.

RAF air gunners didn't train alongside pilots, and weren't trained to fly aircraft. Where pilots wore a pair of wings as their flying badge (or 'brevet') other aircrew wore a single wing denoting their trade, e.g. 'N' for navigator, 'O' for observer and 'AG' for air gunner.

The inter-war RAF, considerably scaled back in size as it was, rather neglected non-pilot aircrew trades; those roles were to be carried out either by pilots, or (particularly for air gunners) volunteer ground tradesmen. Ideally air gunners would attend an air gunnery course, but this was not universal with some (especially overseas) having fairly rudimentary 'on the job' training. This was not a particularly sustainable approach, especially with the massive expansion of the RAF in the 1930s and increasing specialisation of air trades, so from the start of 1939 a scheme was introduced under which all aircrew would be full time positions, though it took some time for this to produce sufficient gunners. On early Bomber Command operations aircraft often flew with volunteer ground crew gunners, the additional pay for air duties being a key motivation for many - actress Minnie Driver's father, for example, was a fitter/rigger with IX Squadron who won the Distinguished Flying Medal flying as a front gunner during the Battle of Heligoland Bight in 1939.

Defiant air gunners would at least have been trained at air gunnery schools, that being the primary purpose of the aircraft, then posted to their squadrons where they would be teamed up with pilots. The training was quite separate, during the aerial components of gunnery training the pilots that gunners flew with would be part of the instructional units rather than trainee pilots.