The Messerschmitt Bf.109 was a single-seat monoplane fighter used by the Luftwaffe before and during WWII. It was used as a fighter, interceptor, high-altitude interceptor, fighter-bomber and reconnaissance aircraft.
The Bf.109G "Gustav" was the most mass-produced variant of the 109, powered by the Daimler-Benz DB 605. The 605 was basically a DB 601 with the cylinder block redesigned to increase displacement from 33.9 to 35.7 litres, which resulted in 175 extra HP for no significant change in size. The Gustav also received improved armament. Instead of the previously standard 7.62mm MG 17 machine guns, 13mm MG 131 heavy machine guns were used. The heavier guns lead to distinct cowling bulges, needed to cover the breechblocks on the new guns.
However, the increased equipment led to heavier weight on the Gustav, in fact 10% heavier than the Friedrich. Compared to the much earlier Bf.109B, the Gustav was almost 46% heavier. The new aircraft had begun to reach front-line units by May 1942. In essence, the Bf.109 design had reached its peak, and would ideally need to be replaced with newer, more modern designs. However, the troubled Me.109 replacement was still in development and things at the front were beginning to look ominous for Germany. Consequently, the RLM decided to continue to further refine the 109.