Avoiding the irrelevancies of the "is the Lysander a bomber" and such other debates, to reinforce the British view of bomber defence prior to WW2, here are some passages from a few important bomber specifications issued.
B.12/36 was for a heavy bomber and was dated 15 July 1936 and contained the following regarding defence: "The aircraft must possess high performance but at the same time be strong in defence against attack from any direction."
Under a separate heading of Armament: it states the following: "Two guns in a power operated turret. Two guns amidships in a semi-streamlined and retractable turret beneath the fuselage. Four tail guns in a power-operated turret."
The Short Stirling won a production order to B.12/36.
P.13/36 was for a medium bomber and was dated 8 September 1936 and contained the following regarding defence: "...all round defence is essential. Power operated gun-turrets are to be located in the nose and tail."
Under a separate heading of Armament: it states the following: Two forward guns in a power operated turret under the control of the second pilot. Four tail gunsin a power operated turret. 1,000 rounds of ammunition per gun, with a reserve of 6,000 rounds."
The Avro Manchester and Handley Page HP.56 won production orders to this spec, but the HP.56 was rethunk and evolved into the HP.57 Halifax.
These are quite specific defensive armament requirements for aeroplanes in 1936, when the status quo aboard bombers in service in air forces around the world was little more than that which was fitted to the Handley Page HP O/400. By 1939, things on paper changed a little and had become more sophisticated. Dated 6 March 1939, Specification B.1/39 was for a 'Bomber Landplane' and became known as 'The Ideal Bomber'. Within was the following: "It is essential to provide a defensive armament for the bomber comparable with that of future fighters, thus involving a certain sacrifice of bomb load. The design of the aeroplane must be arranged to conform to the size and shape of the turrets required."
Under the heading Armament is the following: "In view of the known limitations of the .303 in calibre machine gun, the 20mm Hispano gun has been selected as the weapon to be mounted in the turrets. The best arrangement is to provide two large turrets amidships, one above and one below, each mounting four Hispano 20mm guns. The turrets should be operated by hydraulic or hydro-electric power, the maximum size being taken as 10 ft diameter and a depth of 6 ft. 40 magazines (30 rounds each) of ammunition for each turret shall be provided."
Between 1936 and 1939, when the first power turrets began appearing on modern (excluding the Overstrand) service aircraft, the Blenheim, Whitley and Wellington, the RAF's attitude to gunnery was changing. It had to; the officially held premise was that bombers would avoid losses to enemy fighters by their strong defensive armament by day or by operating at night, which was how BC went to war in 1939 - with precisely that approach firmly in place (see Max Hastings Bomber Command). The Air Gunner role as an RAF trade was just emerging, and a central training establishment for gunnery as favoured by the C-in-C BC, Edgar Ludlow-Hewitt was not formed in October 1939, by which time the war had begun. Ludlow-Hewitt wrote a paper in March 1939 after examining the state of BC at that time and it was scathing to say the least, particularly with regards to gunnery, in which results from target practise in operating the new turrets were dismal, principally because of inadequate training and the difficulty trainee gunners had in operating their turrets. (John Terraine, The Right of the Line) In May 1939, Ludlow-Hewitt wrote to the Air Ministry the following:
"There is little doubt that the weakest point of our bomber force at this moment lies in its gun defence... I fear that the standard of efficiency of air gunners and their ability to resist hostile attack remains extremely low."
At least he was a realist. Whilst he eventually got his wish for a central gunnery school, and the RAF under Harris began to improve accuracy with the addition of advanced navigation and bombing aids - another of Ludlow-Hewitt's bugbears about the contemporary pre-war BC, he then added the following, which was, promptly ignored:
"Experience in China and Spain seems clearly to indicate that with the aircraft in use in these theatres of war at present , Fighter Escorts are considered absolutely essential for the protection of bomber aircraft. So far as I am aware this policy runs counter to the view long held by the Air Staff."