Supermarine Spitfire. At Royal Air Force's service. Part five. After the Battle of Britain.

 

The Spitfire Mk. II was, externally, identical to the Mk. I. It incorporated all the improvements made to the Mk. I. It was powered by the Merlin XII engine rated at 1.175 hp, which employed 100 octane fuel, instead of the 87 octanes one. The Mk. II was manufactured at Castle Bromwich, Birmingham with the first airframe being completed in June 1940 and the first 750 Mk. IIs were equipped with the "a" type wing, while the remaining 170 had the "b" type wing. Cockpit protection was improved and self-sealing fuel tanks were also introduced, albeit many Mk. Is were already retrofitted with them.
Some few experiments were carried out on Mk. IIs, specially aimed to increase Spitfire's operational range of 395 miles (636 km). This was done because the Spitfire was designed as a home-based defence fighter with the smallest possible airframe, so it had little space for extra fuel tanks. Therefore, various external fuel tank configurations were tested, including a 40 gal (151 L) tank faired into the wing leading edge. The Spitfire Mk. II (LR), as it was denominated (LR stands for 'Long Range'), was employed by Nos. 66, 118 and 152 squadrons for a few operations. However, the fact that the tanks couldn't be jettisoned and had an adverse effect on handling the aircraft, caused them to be withdrawn in favour of the fuselage 'slipper' tank.
The Spitfire Mk. II was also the first Spitfire to carry bombs, although of the non-lethal smoke ones. When the last fifty-two examples became Mk. IICs (the letter "C" meant a change in role instead of a wing configuration). These Mk. IICs were officially known as 'Sea Rescue Type E' and carried a small dinghy boat and food supplied to be parachute-dropped. They were employed by five squadrons and redesignated as ASR (Air Sea Rescue) Mk. IIs in late 1942.

After the end of the Battle of Britain, Royal Air Force's (RAF) Fighter Command put to rest many of the squadrons that carried the bulk of the fight and prepared for a campaign of offensive sweeps across the Channel. During this period of time, the Hawker Hurricane was gradually phased out and, in many cases, replaced by the Spitfire. For those fighter sweeps, day fighter squadrons developed a framework of code-names, the most important of which were "Rhubabr" - small scale attacks by fighters of fighter-bombers; "Circus" - heavy escort to light bombers which acted as bait to attract enemy fighters into the battle; "Ramrod" - just like Circus but with the main objective of destroying the bombers' target; "Roadstead" - an attack by bombers with fighter escort on shipping targets; "Rodeo" - a straightforward fighter sweep over enemy territory and finally "Ranger" - a freelance penetration of enemy airspace in squadron or wing strength with the objective of wearing down the defences. 
Such operations were risky for both men and machines and, although the Spitfire squadrons were able to hold against the German Messerschmitt Bf.109E and F variants, the advent of the Focke-Wulf Fw.190 in autumn 1941 and the many ground anti-air fire of all calibres, changed the odds. In fact, many Spitfires were lost to ground fire of all calibre, ranging from rifle rounds to purpose-built flak guns. Among those victims were 'Paddy' Finucane and Bob Stanford-Tuck, aces of the Battle of Britain.
During early 1941, the Spitfire was equipping most operational fighter squadrons and every effort was made behind the scenes, to ensure that it remained at least equal to its Luftwaffe adversaries. Nonetheless, it was undesirable to disrupt production of the Spitfire by introducing radical changes, so a new mark, Mark V was put underway. (To be continued)

Sources:
1st Signal Squadron - Aircraft In action 1039 - Supermarine Spitfire in Action
2nd https://en.wikipedia.org/wiki/Supermarine_Spitfire_operational_history#European_offensive_1941–1943