The Bf 109K was the last of the series to see operational duty and the last in the Bf 109 evolutionary line. The K series was a response to the bewildering array of series, models, modification kits and factory conversions for the Bf 109, which made production and maintenance complicated and costly – something Germany could not afford late in the war. The RLM ordered Messerschmitt to rationalise production of the Bf 109, consolidating parts and types to produce a standard model with more interchangeable parts and equipment; flaws in the design of the airframe were also to be remedied. Work on the new version began in the spring of 1943 and the prototype was ready by autumn. Series production started in August 1944 with the K-4 model, due to changes in the design and delays with the new DB 605D powerplant. The K-4 was the only version to be mass-produced.