Originally the aircraft was designated as Bf 109 by the RLM, since the design was submitted by the Bayerische Flugzeugwerke (literally "Bavarian Aircraft Works", meaning "Bavarian Aircraft Factory"; sometimes abbreviated B.F.W., akin to BMW) during 1935. The company was renamed Messerschmitt AG after July 11, 1938 when Erhard Milch finally allowed Willy Messerschmitt to acquire the company. All Messerschmitt aircraft that originated after that date, such as the Me 210, were to carry the "Me" designation.

Despite regulations by the RLM, wartime documents from Messerschmitt AG, RLM and Luftwaffe loss and strength reports continued to use both designations, sometimes even on the same page.

Sources:
Gunston, Bill - The Encyclodepia of the Worlds Combat aircraft, 1976, Chartwell Books, Inc., New York
Brown, Eric, Captain - Wings of the Luftwaffe, 1979, Airlife Publishing Ltd., Shrewsbury
Gunston, Bill & Wood, Tony - Hitler's Luftwaffe, 1977, Salamander Books Ltd., London
Donald, David - The Complete Encyclopedia Of World Aircraft, 1997, Brown Packaging Books Ltd., London
Scutts, Jerry - Messerschmitt BF109: The Operational Record, 1996, Airlife Publishing Ltd, UK
Wikipedia - BF 109