Messerschmitt Bf.109E, German users, part two

During late 1938 the Bf.109E entered into production. Engine was changed from the Junkers Jumo 210 to the larger Daimler-Benz DB601A engine which yielded much power despite being heavier. Due to the need of a larger area to disperse the extra heat generated by the engine, a redesign of the fuselage was needed, so as enlarging the nose radiator would've created an excessive negative drag, it was decided to place them beneath the wings, just in the juncture between wing root and wing pannel, leaving the oil cooler under the nose in a small streamlined duct. The new radiator position also contributed to counterbalance the extra weight of the engine which drove a three-bladed propeller.
In order to fit the new radiators, wings were almost completely redesigned and reinforced, with several inboard ribs cutted down to make place for the radiator ducts. Due to the new radiator configuration, cooling now was much more efficient, in spite of the extra weight and extra ducting and piping which could be too vulnerable in combat. Furthermore, undercarriage, when deployed, could throw up mud and debris on wet airfields that could clog radiators.
To test the new DB601A engine rated at 1100PS (1085hp 809Kw) two more prototypes, V14 and V15 were built, each of them with different armament, as the V14 was armed with two 7.92mm (0.312in) placed above the engine in a cowl, and one 20mm MGFF in each wing. The V15 was armed just with two 7.92mm placed above the engine. After flight tests, the V14 proved to be more promising and a pre-production batch of 10 Bf.109E-0 was ordered into production. Both early variants of E-1 and E-3 were shipped to Spain to be evaluated in combat during the late stages of the Spanish Civil War.

Sources:
1. https://en.wikipedia.org/wiki/Messerschmitt_Bf_109_variants#Bf_109E
2. Salamander Books - The Complete Book of Fighters
3. Osprey - Air Vanguard 23 - Messerschmitt Bf 109 E-F series(1)
4. Signal Squadron - Aircraft In action  44 - Messerschmitt Bf-109 in Action Pt.1