The Bertha's airframe was very similar to that of its predecessors. It was a streamlined design, with its canopy merging directly into its fuselage. While this meant limited rear visibility, it also made a superior aerodynamic silhouette.
The Bertha featured a number of new technologies for its time. Its retractable landing-gear and enclosed canopy were novelties that went on to become standard of all aircraft in later years.This advancement came with a price, however. Pilots that were used to fixed landing-gear often forgot to bring the wheels up or down for take off and landing. This resulted in a large amount of landing accidents. Approximately 1,600 Berthas were lost during failed or flawed landing attempts. The undercarriage was also known to collapse under stress, furthering the risks of landing the craft.
The fighter retained the twin-blade fixed wooden propeller of the previous models, but was upgraded from the 631 horsepower Jumo 210B engine to the 680 horsepower Jumo 210Da engine. 
The Bertha was outfitted with a ReVi reflector sight and a FuG 7 radio in standard format.
The armament of the Bf 109B was, at best, underwhelming. The original design called for three MG 17 machine guns in the nose; two in the upper cowlings, and one in the propeller's nose cone. However, the engine-mounted machine gun was unreliable and detrimental to the plane itself. This left the craft with only two rifle-caliber light machine guns which, compared to its contemporaries, left much to be desired.
Performance wise, the Bertha was groundbreaking for its time. Its top speed of 467 km/h surpassed that of its common opponents (the I-15 "Chato" at 350 km/h, and even the newer I-16 series 5 "Mosca" at 445 km/h), and its altitude threshold was much more lenient. 
Due to its weight, the BF 109B was exceptional at diving and energy retention. Its high-altitude characteristics were phenomenal when compared to its rivals, although at low-altitude the Republican fighters could easily out-maneuver it.
After already supplying the Nationalist forces with a scarce amount of the new Bf 109 V's and A's prototypes, it was clear that Germany was devoted to the Nationalist side of the war. Satisfied with the 109's performance, the Reichsluftfahrtministerium (German Ministry of Aviation) ordered a version of the monoplane with the latest Jumo engine, designating it the Bf 109B. 
The Germans were already utilizing the Spanish conflict as a combat testing grounds for their new technology, and as such the first batch of 109Bs were almost immediately shipped to Spain upon completion in 1937.The all-German volunteer Condor Legion made ample use of the 109s, finding them superior replacements to the older He 51 and Ar 68E biplanes. A the time of its introduction, the Messerchmitt Bertha was one of the most advanced fighter aircraft designs in the world, unmatched by any of its adversaries.
Later in the its service, however, the Bertha's superiority began to fade. By the end of 1937, the Messerschmitts were completely outclassed by the new Polikarpov I-16 Type 10 "Supar Mosca" fighter in nearly every aspect. The older Bertha was meant to be supplemented by the "Clara" series of 109s in early 1938, but these newer models arrived in small numbers. As the Berthas began to suffer losses, the Condor Legion began to run out of aircraft. 
The remaining Berthas served in the Spain until they were fazed out of service by the completion of the "Dora" series in the summer of 1938, and were rendered completely obsolete by the introduction of the "Emil" in 1939. 
Bf 109B-1: the original variant with the standard wooden propellers
Bf 109B-2: a later variant with metal, variable-pitch propellers. Many unofficial B-2s were retrofitted from B-1s.
Bf 109 V10 - V13: four V-model prototypes were constructed on the 109B's airframe. These were fitted with the Daimler-Benz DB 600A engine to increase performance. Another engine, the DB601A, was used instead for the following models.
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 http://acepilots.com/german/bf109.html
- ↑ 2.0 2.1 2.2 2.3 http://bioold.science.ku.dk/drnash/model/spain/bf109.html
- ↑ 3.0 3.1 3.2 3.3 http://www.airpages.ru/eng/lw/bf109bcd.shtml
- ↑ 4.0 4.1 4.2 http://www.historyofwar.org/articles/weapons_bf_109B.html