Messerschmitt Bf.109G, German users, part one

The Bf.109G series was the previous Bf.109F which was identical to but it differed in having a reinforced wing structure, an internal bullet-proof windscreen, it used heavier welded framing for the cockpit transparencies, and had additional light-alloy armour for the fuel tank. Originally it was going to have small doors to cover the outer portions of the wheels when retracted and, in order to incorporate it, the outer wheel bays were squared off. Two small additional inlet scoops for additional cooling of the spark plugs were added on both sides of the engine cowling. It also had omitted the boundary layer bypass outlets on the upper radiator flaps, which had been very characteristic of the Bf.109F series.
Like most of the German fighters of the World War 2, it was designed to be highly versatile, and therefore it allowed many field modifications, such as the inclusion of additional fuel tanks for long-range flights or the inclusion of bombs to fullfil the fighter-bomber role. Special high-altitude interceptor variants were also produced.
It was fitted with a Daimler-Benz DB 605A engine, which was a development of the DB 601E which powered the previous Bf.109F. The DB 605A incorporated improvements in displacement and compression ratio, as well as other improvements that made its manufacturation easier. It yielded a power of 1455hp (1085Kw) and, initially suffered from reliability problems as it power output of 1475PS made the engine to overheat excessively so the Luftwaffe banned that output and set it at 1200PS. The full output wasn't reinstated until 8th June 1943 when Daimler-Benz issued a technical directive.
Earlier versions resembled the F-4 variant, and even carried the same basic armament and from 1943 onwards, bulges in the cowling were added when the 7.92mm MG 17 were replaced with 13mm MG 131 (from the G-5 onwards), due to larger breechblocks and on the wings due to larger tyres. It continued to be improved with better cockpits, greater firepower and better engine modifications.

Sources:
1. https://en.wikipedia.org/wiki/Messerschmitt_Bf_109_variants#Bf_109G
2. Salamander Books - The Complete Book of Fighters