Messerschmitt Bf.110. Prototypes and early variants.

Through the 1930s, the air forces of the major powers were on a race to transition from biplanes to monoplanes designs. Most of them concentrated their efforts on single-engined fighters, but the problem with the range arose. The RLM (Reich Aviation Ministery), leaded by Hermann Göring, issued a request for a new multipurpose fighter called Kampfzerstörer (Battle Destroyer) with long range and internal bomb bay. The request called for a twin-engined, three-seat, all-metal monoplane that was to be armed with cannons and had to have a bomb bay. Of the seven companies the requirement was issued to, only three of them, Bayerische Flugzeugwerke (Messerschmitt), Focke-Wulf and Henschel accepted the request.
Messerschmitt defeated their competitors and was assigned the funds to build various prototype aircrafts. In order to increase the armament element of the RLM specification, Messerschmitt skipped the bomb bay requirement and the prototype of the Bf.110 proved to be far superior to its competitors in speed, range and firepower. By the end of 1935, the prototype of the Bf.110 had evolved to be an all-metal, low-wing cantilever monoplane of semi-monocoque design with twin vertical stabilizers and powered by two Daimler-Benz DB 600A engines and was also fitted with Handley-Page wing slots which were leading-edge slats.
Due to Ernst Udet pressures and by sheer luck, the RLM reconsidered the ideas of the Kampfzerstörer, and began to focus on the Zerstörer and, thanks to those specifications on the changes, Bayerisches Flugzeugwerke's design better fitted RLM requirements. On 12 May 1936 Rudolf Opitz flew the first prototype at Messerschmitt factory in Augsburg, close to Munich in Bavaria. However, like many other pre-war designs found, the engine technologies promised were not up to acceptable reliability standards. Even with the DB 600 engines, the RLM found that the Bf.110, while not as maneouvrable as desired, it was much faster as the original request specified, as well as faster than the Bf.109B, Luftwaffe's standard frontline fighter back then. Therefore, a pre-production order of four Bf.110A-0 was placed in January 1937 meaning that the RLM rejected both Henschel and Focke-Wulf designs.
The Bf.110A was apre-production aircraft with just 4 exemplars built. It was powered by two Daimler-Benz DB 601 engines and was armed with just the four nose-mounted 7.92mm (0.312in) MG.17 machine guns.
Due to delays with DB601 engine deliveries Bayerische Flugzeugwerke was forced to switch their powerplants for their subsequent Bf.110B variant, which was powered by two Junkers Jumo 210B inverted V design, 12-cylinders and were rated at 690hp (515Kw). This engine choice made the Bf.110B seriously underpowered and able of reaching a top speed of jsut 431 Km/h (268mph).
Even without the DB.601 engines, Bayerisch Flugzeugwerke continued manfacturing the Bf.110 from mid 1937 onwards using the Junkers Jumo 210G engines for the B-1 variant which was armed with four 7,92mm MG.17 machine guns in the nose and two defensive 20mm MG.FF. The B-2 was a reconnaissance version with the nose guns replaced by cameras, and the B-3 which was used as a trainer, with the cannons replaced with extra radio equipment. Only 88 Bf.110B were built from July 1938 until December 1938 when the DB.601 engines became largely available and production switched towards that variant.The B variant was deemed obsolete by the beginning of the World War II was used as trainers. However, its possible that some few of them served briefly during the early stages of the Battle of France in May 1940 with the ZG 26.

Sources:
1. https://en.wikipedia.org/wiki/Messerschmitt_Bf_110
2. Salamander Books - The Complete Book of Fighters
3. https://forum.valka.cz/topic/view/6516