Messerschmitt Me-109 V-21 (Source: Die Deutsche Luftwaffe, Willy Messerschmitt)

It is not known for certain why Messerschmitt started the development of a radial engined 109. One theory is that in order to open up more export markets a 109 was developed that didn't utilize the strategically important DB 601 engine. Thus a version using the BMW 139 was developed. Another theory is that Messerschmitt wanted to compete with the FW-190 then in development and wanted to offer a fighter using the same BMW 801 engine. Only because this was not yet ready was a BMW 139 engined aircraft designed first.

What is for certain is that in 1938 the V-21, used for the development of the E-series was modified to take a BMW 139 engine, a copy of the Pratt & Whitney Twin Wasp. A new forward fuselage was designed from frame no.6 forwards, enabling the fitting of the radial engine. A new full vision "bubble" canopy was fitted, similar to the canopy of the Me-262. The aircraft flew in 1939.

X-0 (1940)
The second radial engined 109 was a modified F-1 airframe, fitted with a fuselage similar to the V-21 with the BMW 139 engine. The Me-109X used the BMW 801 however. The aircraft flew in September 1940 and was tested by the RLM. The RLM decided however that the FW-190 was performing well and that there was no need for a similar powered fighter. Therefore the program was ended in 1941.

Messerschmitt Me-109 V-21 (Source: AJ Press Monografie: Messerschmitt Me-109 Cz. 3)

Messerschmitt Me-109 X (Source: AJ Press Monografie: Messerschmitt Me-109 Cz. 3)