Messerschmitt Bf.109B/C/D - Spanish Civil War, part one

The Messerschmitt Bf.109B was the first mass produced variant and it was powered by 661hp (493Kw) Junkers Jumo 210D engine with a two-bladed propeller. Most of the previous Bf.109As were upgraded to the Bf.109B variant. It featured a variable-pitch propeller with the expectative of fitting an MG.17 that would fire through the spinner, however it's not clear if that machine gun was fitted in any of the aircraft that served in the Spanish Civil War. It had also three extra large cooling slots in the top and in the bottom of the forward cowling to increase the cooling of both engines and machine guns. In later machines underwing oil cooler was placed slightly further forward. This variant featured also shorter wing slats.
The next variant, the Bf.109C was powered by a 690hp (515Kw) Junkers Jumo 210G engine with direct fuel injection, increasing that way the performance at high altittudes, two extra machine guns fitted in the wings, exhaust ejector stubs were projected from the sides of the cowling, rather than being flush, like in the previous model, and both oxygen filter and electrical socket were moved back from under the cockpit to further back in the fuselage of the starboard side. This variant was the least produced of them all with very few of them serving in the Condor Legion from spring 1938 until the end of the Spanish Civil War.

Sources:
1. http://bioold.science.ku.dk/drnash/model/spain/bf109.html
2. https://en.wikipedia.org/wiki/Messerschmitt_Bf_109_variants#Bf_109_A.2FB.2FC.2FD
3. https://en.wikipedia.org/wiki/Messerschmitt_Bf_109_operational_history
4. Signal Squadron - Aircraft In action 44 - Messerschmitt Bf-109 in Action Pt.1