Arado Ar.68E - Prototypes and prewar German users

The Arado Ar.68 was a single-seat biplane fighter of the mid-1930s and it was one of the first fighters massively produced by the German industry when they abandoned the restrictions of the pact of Versailles.

It was developed in parallel to the Ar.67 and many prototypes of it were made:

• Ar.68V1, AKA Ar.68a: The very first prototype, it was powered by a 750hp BMW VId engine. It flew for the first time in 1934. Unarmed.
• Ar.68b/c: The second and third prototypes. Both of them were powered by a 680hp Junkers Jumo 210A 12-cylinder inverted-vee liquid-cooled engine. The b variant was unarmed, while the c variant was armed with two 7,9mm MG 17 machine guns with 500 rounds each.
• Ar.68d, AKA Ar.68V4: Production prototype powered by a 650hp BMW VI engine.
• Ar.68e, AKA Ar.68V5: Production prototype powered by a 660hp Junkers Jumo 210Da engine. 
• Ar.68E: The first type to enter service with the Luftwaffe, it was powered by a 610hp Junkers Jumo 210.

Sources:
1. https://en.wikipedia.org/wiki/Arado_Ar_68
2. Salamander Books - The Complete Book of Fighters
3. http://histaviation.com/ar_68.html