Early in 1940 the Augsburg Messerschmitt factory began work on improving the aerodynamic shape and streamlining of the design in order to obtain maximum performance. Four prototypes were built from the E-4 production run, each intended to test one or more of the changes.

The first change was to the shape of the nose with a steady even slope down to the spinner replacing the “stepped” look of the Emil. The spinner was enlarged and rounded to provide a smooth aerodynamic shape. Finally, the supercharger air intake was changed from square to round and was placed away from the fuselage to improve the ram effect.

The tail section saw a reduction in the size of the rudder from 8.1 to 7.5 sq. ft. and the horizontal stabilizer was moved slightly forward and down, with the elimination of the support struts being the most visible change. This later proved to be a problem when sympathetic vibrations from the engine caused metal fatigue to set in, which resulted in failure of the fuselage just forward of the tail. Installation of external braces in the field fixed the problem for existing planes while production changes were made to improve structural strength.

The largest drag reduction came from a redesign of the under-wing radiators, which were both reduced in height and were recessed into the wing. A boundary layer bypass system was devised to collect turbulent air in front of the radiators, leading it over the radiators and out through a duct in the inner flaps.

The V-21 and V-22 prototypes had the wingspan reduced by two feet but this had a detrimental effect on handling. The V-23 prototype installed a pair of detachable, semi-elliptical wingtips that restored all but 2.16 sq. ft. of the original wing area. This worked well and the wingtips were incorporated as a standard part of the wing on all subsequent versions.

Initially intended to use the 1,375 hp DB-601E, the engine wouldn’t be ready for a full year after F-series production commenced. So in the meantime Messerschmitt used 1,200 hp DB-601N, which achieved its performance partly through the use of higher compression ratios and 96 octane fuel.

Similarly, the plane’s armament was to be considerably changed from that of the Emil. Replacing the pair of wing-mounted MG-FF/M cannon was to be a single MG-151 cannon, which would be produced in both 15mm and 20mm versions. The higher rate of fire, greater hitting power and higher ammo loads were considered to provide an overall greater firepower. Additionally, removal of the wing-mounted guns improved roll performance, which was a welcome change to pilots. However, the MG-151 wasn’t yet ready and a single engine-mounted MG-FF/M was used instead.

This version went into production as the F-1 with the first examples rolling off the assembly line in November 1940. By March and April 1941 JG 2 and JG 26 on the channel coast were converting in large numbers to the F-1 and the newer F-2s, which utilized the 15mm MG-151, the 20mm model not yet being ready.

At the beginning of 1942 the F-3 started coming off the production line using the now available DB-601E. The lower compression ratio of the 601E allowed 87-octane fuel to be used, which was more readily available. Simultaneous with F-3 production was the F-4, which utilized the heavier hitting 20mm MG-151, with the ammo load being reduced from 200 to 150 rounds. Further improvements included additional self-sealing to the fuel tanks and additional pilot armor. The F-4/B added the capability of carrying bombs or a drop tank, while the F-4/Z utilized an experimental nitrous-oxide injection system.

A number of F-4s were used to experiment with various engine and landing gear installations. These included a tricycle landing gear system as well as experiments in using the BMW-801 radial and Junkers Jumo 213 inline with annular radiator (the same as used in the FW-190D).

Despite the increased weight of the Friedrich over the Emil, the performance of the Friedrich was considerably better. Many pilots considered the Friedrich to be the zenith of 109 development, with the best combination of speed, firepower and turn-rate.