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 ill-afford late in the war. The RLM ordered Messerschmitt to rationalise production of the Bf 109, consolidating parts, types, and so on, to produce a uniform, standard model with better interchangeability of parts and equipment. At the same time, the existing flaws of the design were to be remedied. Work on the new version began in the spring of 1943, and the prototype was ready by the autumn of that year. 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.
Externally the K series could be identified by changes in the locations of the radio equipment hatch, which was moved forward and to a higher position between frames four and five, and the filler point for the fuselage fuel tank, which was moved forward to a location between frames two and three. In addition, the D/F loop was moved aft to sit between frames three and four on the top fuselage spine and a small circular plate above the footstep on the port side of the fuselage was deleted. The rudder was fitted as standard with a Flettner tab and two fixed tabs although some rare examples were not fitted with the fixed tabs. All K-4s were to be fitted with a long retractable tailwheel (350 × 135 mm/14 × 5 in) with two small clamshell doors covering the recess when the tail-wheel was retracted.
The wings featured the large rectangular fairings for the large 660 × 190 mm (26 × 7 in) main wheels. Small wheel well doors, originally planned for the G series, were fitted to the outer ends of the wheel bays, covering the outer wheels when retracted. These doors were often removed by front-line units. The ailerons were fitted with small, adjustable trim tabs. The radio equipment was the FuG 16ZY with an antenna mast fitted under the port outer wing and FuG 25a IFF as well as the FuG 125 Hermine D/F equipment. Internally, the oxygen bottles were relocated from the rear fuselage to the right wing. Flettner tabs for the ailerons were also to be fitted to serial production aircraft to reduce control forces, but are only seen on photos of later production aircraft.
Armament of the K-4 consisted of a 30 mm (1.18 in) MK 108 engine-mounted cannon (Motorkanone) with 65 rounds, and two 13 mm (.51 in) MG 131s in the nose with 300 rpg although some K-4s were fitted with the MG 151/20 as the Motorkanone. Additional Rüstsätze, or equipment kits, such as a 300 L (80 US gal) drop tank (R III), bombs up to the size of 500 kg/1,100 lb (R I), underwing 20 mm Mauser MG 151/20 cannon gondola pods (R IV) or 21 cm (8 in) Wfr.Gr. 21 rockets (as on the Gustav models) could be carried after minimal preparations; the latter two however were rarely used by Bf 109 units at this stage of the war, but there is evidence that III./JG 26 were almost completely equipped with K-4s which were fitted with R IV. In addition there were problems with the 30 mm (1.18 in) MK 108 Motorkanone:
Power was provided by a Daimler-Benz DB 605DM (early models - engine produced in limited numbers) and the DB 605DB/DC engine (production variants of the DM) with an emergency power rating of 1,600/1,800 PS at 6,000 m (1,160 PS maximum continual at 6,600 m), and take-off power of 1,800/2,0000 PS at 0 m. The DB and DM used MW50 methanol-water when running on B4 fuel, while the DC required C3 fuel with MW50 usage. A wide-chord, three bladed VDM 9-12159 propeller of 3 m diameter was used, as on the G-6/AS, G-14/AS and G-10.The 30mm cannon were extremely potent weapons, but they had a tendency to jam, and apparently all of the K-4s supplied to III./JG 26 were also equipped with 20 mm-guns in the hated underwing tubs. Uffz. Georg Genth's regular aircraft was a G-10, but on occasion he flew a K-4. He preferred the G-10 as a dogfighter, as the K-4's bulky armament sharply reduced its manouevrability.
Deliveries began in mid-October 1944. 534 examples had been delivered by the Messerschmitt A.G., Regensburg by the end of November 1944, and 856 by the end of the year. Regensburg delivered a total of 1593 by the end of March 1945, after which production figures are missing. With such a high rate of production, despite continuous heavy fighting, by the end of January 1945 314 K-4s – about every fourth 109 – were listed on hand with the 1st line Luftwaffe units. Ultimately it was intended to equip all Bf 109 units with the 109K, which marked the final stage of 109 development before the jet age.
The Bf 109 K-4 was the fastest 109 of World War II, reaching 710 km/h (440 mph) at 7,500 m (24,610 ft) altitude; improved propellers were being developed when the war ended which would have boosted the speed to 727 km/h (452 mph), and 741 km/h (459 mph) was expected with an experimental swept-back propeller design. The Rate of climb was 2,775 ft (850 m)/min. The standard Revi 16C reflector sight was fitted, which was slated to be replaced later by the EZ 42 Gyro gunsight, although this never happened. The Bf 109 remained comparable to opposing fighters until the end of the war. However, the deteriorating ability of the thousands of novice Luftwaffe pilots by this stage of the war meant the 109's strengths were of little value against the numerous and well-trained Allied fighter pilots.
Several other versions were projected based on the 109K airframe – K-6, K-8, K-10 and K-14. In the proposed K-6 the armament would have been two 13 mm (.51 in) MG 131 above the engine, along with a 30 mm (1.18 in) MK 108 Motorkanone and an internally mounted MK 108 in each wing, with 45 rpg. Alternatively, the wing MK 108s could be substituted by 20 mm MG 151/20s, with 100 rpg. Armour weight was increased to 200 lb (91 kg). Takeoff weight was 7,986 lb (3,622 kg). Some K-6 prototypes were built and tested at the Erprobungstelle Tarnewitz weapons-testing centre on the Baltic coast.
Project drawings of the K-8 show an K-series airframe powered by the two-stage DB 605L high altitude engine, a high-velocity 30 mm (1.18 in) MK 103mot Motorkanone, and two 30 mm (1.18 in) MK 108 cannons in the wings; the cowl 13 mm (.51 in) MG 131s were dispensed with.
Some sources point to limited use of the K-14, intended as high-altitude heavy fighter. Two airframes are listed as delivered to II./JG52 under Major Wilhelm Batz in late spring of 1945, these being armed with only one 30 mm (1.18 in) cannon, but the type's existence cannot be positively confirmed. The K-14 was to be powered by the two-stage supercharged DB 605L engine, the use of a four-bladed propeller 460 mph (740 km/h), and an operational altitude of 38,000 ft (12,000 m) was projected. Armour and armament were otherwise similar to the K-6.