The spar — the large structural beam upon which the wings were built — was made up of box-like girders that fitted one inside the other. Each was a different length, so that although the girder had five layers near the wing root where the stress was greatest, it was hollow at the wingtip where there was less strength needed. The spar was like a huge leaf-spring.

Forward of the wing spar the wing was covered with heavy-gauge metal, so that the whole leading edge of the wing was, in effect, a box. This gave the wing immense strength. Aft of the spar the wing was clad in thinner metal.

The Supermarine factory wasn't big enough for even the first modest order of 310 aircraft, so they had to subcontract the pieces. One company made wingtips, another made wing leading edges, another made engine mountings, etc. At Supermarine's they put the pieces together and tested them. There were many teething troubles but considering how much finer were the tolerances for this sort of aircraft, it was a miracle that such a method worked at all.

The Spitfire's curved wing is said to have been influenced by the Heinkel He 70. Whether or not this is specifically true, Mitchell, along with the rest of the world of aviation, must have found Heinkel's superb combination of aesthetic line and aerodynamic efficiency inspiring.

It was in the spring of 1935 that the demand for four extra machine guns forced Mitchell to abandon the orthodox tapering wing. The new elliptical wings that made the Spitfire so distinctive in the air were a logical response to keeping the machine-gun breeches in line while still having space to house the wheels without bumps or bulges.

The production difficulties caused by the subtle wing shape were more than outweighed by the advantages. Not only did the curved shape provide maximum area and low wing loading for a given span, but the long cord gave strength at the wing roots where it was fixed to the fuselage. All the time the wing was kept as thin as possible. By the standards of the day the result looked more like a racer than a military plane. Amazingly Mitchell's Spitfire wing, housing four big machine guns, was thinner than that of the Messerschmitt. It was an inspired achievement, for only long after Mitchell's death, as speeds went up and up, did research show how far ahead of its time Mitchell's wing design had been. Vickers suggested to the Air Ministry that the name Spitfire used for the previous gull-wing prototype, should be kept. Mitchell was not consulted on this point, but neither was he surprised. Told of the decision, he said it was "just the sort of bloody silly name they would choose."

But Mitchell was proud of his aeroplane. The two test pilots, flying it at Eastleigh during the early summer of 1936, often noticed Mitchell's old Rolls-Royce motor car parked alongside the hangar. He had come to watch it fly. It was just as well, for he never lived to see his fighters coming off the production line. In 1937 he died aged 42.

Both the RAF fighters had eight Browning machine guns. The Bf 109E was equipped with two 7.92-mm machine guns and two 20-mm cannon.[2] Although exact comparison is difficult (because in combat the 20-mm cannon were firing thin-shelled 'mine type' missiles that exploded on impact), it is reasonable to say that a three-seconds' burst of gunfire from the Messerschmitt Bf 109E weighed 18 lbs. The RAF fighter fired nearly 13 lbs in the same duration of shooting.

The Bf 109E's 20-mm Oerlikon cannon (one in each wing) was a Swiss anti-aircraft gun drastically modified for use by aircraft. This meant making it smaller and lighter, with a faster rate of fire. The German experts shortened the barrel and redesigned the 20-mm shells, reducing the amount of powder inside them. This allowed the breech-block mechanism to be lightened.

In some respects the modification was brilliant: the gun was very light and very compact, but the design of the breech-block did not incorporate anything to lock it when the shell fired. The explosion in the breech that fired the shell also opened the breech for the next one. All of this resulted in a very poor muzzle velocity. However, the experts reasoned that since all air combat takes place at close range, and because the missile would actually explode, muzzle velocity was not very important. As we shall see, this proved to be a major error.

Its other shortcoming was the rate of fire (see Table 2).

The German pilots could select their fire: on a stick-like control column a thumb button on top fired the wing guns, and a finger-trigger fired the guns on the cowling. The latter were considered very useful by pilots who sighted along the stream of bullets, while still looking through the gun-sight. The German fighters were equipped with indicators to show ammunition supply. The RAF pilots usually had a couple of dozen tracer bullets at the bottom of the box to warn them that their ammunition was expended.

Browning Oerlikon Rhein me tall-Borsig (RAF) MG FF MG 17

Calibre 7.7 mm 20 mm 7.92 mm (0.303 in)

Supply carried 300 60 1,000' (rounds)

Gun weight 22 53 28 (lbs)

Gun length 44.5 52.8 47.2 (inches)

Muzzle velocity 2,660 1,800 2,450 (f/sec.)

Rate of fire 1,200 520 1,100 rds ming

Projectile weight 0.344 4.82 0.45 (ounces)

When fitted as wing armament only 500 rounds per gun.

Fast in comparison with an anti-aircraft weapon, it was slow by the standards of the fighter pilot, who could seldom hope to have a target in his sights for more than two seconds (seventeen shells from each of the two cannon). And such a two-second burst consumed well over a quarter of the supply of shells, for even with Messerschmitt's wings modified by bumps, there was only a 60-round drum on each cannon.

In fighter-to-fighter combat, the two sides were not far apart in firepower, but the structural strength of the larger aircraft, that is, the German bombers, made it very difficult to bring them down with bullets. This is illustrated by an incident in the Battle when six Spitfires of 74 Squadron expended 7,000 bullets in attacks on a Dornier Do 17 but did not bring it down.

Although the rival merits of machine gun and cannon were much argued at the time, the RAF had secretly concluded that the cannon was far better. In 1940 (at the Royal Aircraft Establishment), a series of tests was carried out against an old Blenheim airframe (incorporating armour). The eight machine-gun configuration was fourth in a list in which two cannon were top.

As an experiment, the RAF used a few Spitfires equipped with cannon during the 1940 battles. However the RAF had trouble with its cannon guns, and the few pilots given them during the Battle of Britain for the most part cursed their luck and were re-equipped with machine-gun fighters.

One of the problems of the Browning machine gun, in RAF use, was the fact that it had to fire the army's rifle bullets. The British army persisted in using nitro-glycerine cordite propellant at a time when virtually every other army had changed to the nitro-cellulose type. Whatever the rival merits of the two types of ammunition, cordite, for instance, was more stable — in tropical conditions the cordite bullets were apt to detonate in the breech of hot guns. There could hardly have been a graver disadvantage from the fighter pilot's point of view. The Browning was modified to overcome this problem (by holding the breechblock open, with empty chamber, at the end of each burst). It was also modified to suit large-scale mass production, and again for a problem that arose with the feed. By this time there was hardly a component of the gun that remained unaltered. The Colt-Browning gun used in the Battle was virtually a British gun: a rare event for a nation that had used so many foreign-designed weapons, from Lee-Enfields, Maxims, and Lewis guns to Brens, Bofors, and Besas.