Messerschmitt Bf.109 Prototypes - Participation in the Spanish Civil War

The Messerschmitt Bf.109 began its career fighting since the very beginning. In fact, the prototypes, from the V3 onwards, served with the Condor Legion, the German volunteers serving with the Aviación Nacional (Nationalists' Air Force).
That way, the V3, which was quite distinctive thanks to its extra cannon firing through the propeller. It served in the Spanish Civil War with the spinner modified to the V4 standard, as it was better. The V4 and V5 resembled the Bf.109A and they were modified to that standard even if it wasn't an homogeneous version. All prototypes served with the VJ/88 that was the test squadron of the Legion.
The A version seved with the fighter unit of the Condor Legion, the J/88 and many times they have been misidentified with the Bf.109B version. They were marked from 6-1 to 6-16 and served in the early part of the conflict, specially in the northern front. All of them were either modified to the B standard or withdrawn from service. One of them marked as 6-15, ran out of fuel and had to land on Guadalajara, North of Madrid, that was still part of the Republic. It was sent to Sabadell, in Catalonia, to be tested by Republican and French pilots and sent by ship to the USSR where it was furtherly tested and its fate is unknown.

Sources:
1. https://en.wikipedia.org/wiki/Messerschmitt_Bf_109_variants
2. http://bioold.science.ku.dk/drnash/model/spain/bf109.html
3. Salamander Books - The Complete Book of Fighters