A clear problem with early War British fighters was a lack of heavy firepower.

Let’s take a look at the first production model to overcome that shortfall.

Even before the Battle of Britain RAF pilots had complained that their battery of 8 light machine guns was inadequate against larger aircraft, in particular those modern Luftwaffe types with crew armor and self sealing fuel tanks. The large volume of lightweight fire could shred fabric covered surfaces, or poke holes in aluminum skin; but it often did nothing to engines or crew compartments. Combat reports showed an average of 4500 rounds were needed to bring down an aircraft.
The first attempt to improve killing power was replacing the .303s with a single 20 mm Hispano cannon in each wing. 30 Spitfire Mk I were modified this way and field tested by 19 Squadron during the Battle of Britain. But their reliability, a tendency to jam, was so severe the squadron requested that all their Spitfire Ib be replaced with Mk Ia planes.
Meanwhile Supermarine had managed to improve the armament a great deal. A better mount for the guns, and a decision to retain 2 of the light machine guns in each wing made their next attempt with cannon far more successful. This entered production as the Spitfire Mk IIb. Overall, this made the Spitfire a far more effective fighter.

921 Spitfire Mk II were built, 170 of these were the Mk IIb. One source claims these deliveries came very rapidly and Mk II completely displaced the Mk I in combat units by the end of 1940. I’m a little skeptical of this claim (I saw it on Wikipedia, can anyone point me to an actual credible source with information on this?!) The total Mk II production was not large, it overlapped with Mk I production Summer of 1940, and Mk V production from the start of 1941. Which makes me doubt it was ever the sole operational Mark of Spitfire. But stranger things have happened, call it a minor enigma.

This particular aircraft flew with 303 Squadron in the Summer of 1941. That Squadron number is well known! It was the most successful Squadron in the Battle of Britain, and the first to be manned entirely by Polish refugees. Which explains both the old Polish Air Force insignia just below the forward windscreen, and the nose art! The Poles seemed to prefer more colorful mounts than RAF standard.
It was assigned to F/O Miroslaw Feric, an ace with 8 kills. Later, in February 1942, he was killed in action in a Spitfire Mk V. His claim to fame was that he kept a detailed diary from the start of his War flying with the Polish Air Force against the Nazis through all his travels and his action with the RAF. That diary later became the basis for the official 303 Squadron unit history.

This is of course, the Eduard Spitfire again. It is from the Dual Combo boxing called “Spitfire Story: Tally Ho!” and was paired with the Spitfire Mk IIa I posted previously.