Simultaneous with its approval of the resumption of work on the Graf Zeppelin, the Seekriegsleitung also took some concrete action to address the perceived need for additional carriers. The incomplete hull of the heavy cruiser Seydlitz, on which there had been little progress since late 1940, was selected for conversion into a carrier. The superstructure was razed to the upper deck in preparation for erecting a hangar and flight deck but, as with the Graf Zeppelin, all work ceased at the end of January 1943. The hulk was towed to Königsberg and scuttled there in April 1945. The Kriegsmarine never commissioned a carrier.
Following the loss of the battleship Bismarck in May 1941, during which British aircraft carriers proved instrumental, and the near torpedoing of her sistership Tirpitz in March 1942, the Kriegsmarine became convinced of the necessity of acquiring aircraft carriers. Work on the purpose-built carrier Graf Zeppelin, which had been halted in April 1940, was resumed in March 1942. The Kriegsmarine also decided to convert a number of vessels into auxiliary aircraft carriers. Seydlitz was among the ships selected for conversion, along with several passenger liners.
At the same time as construction of Graf Zeppelin resumed, conversion work began on Seydlitz. The majority of the superstructure was cut away, with the exception of the funnel, to prepare for the installation of a flight deck and an aircraft hangar. In total, approximately 2,400 t (2,400 long tons; 2,600 short tons) of material from the ship was removed. The flight deck was to have been 200 m (660 ft) long and 30 m (98 ft) wide. The hangar was 137.5 m (451 ft) long and 17 m (56 ft) wide forward and 12 m (39 ft) wide amidships and aft. Her armament was reduced to an anti-aircraft battery of ten 10.5 cm L/65 guns in twin mounts, two forward of the conning tower and three aft, ten 3.7 cm guns in dual mounts, and twenty-four 2 cm guns in quadruple mounts.
Seydlitz's air complement was to have consisted of ten Bf 109 fighters and ten Ju 87 Stuka divebombers. The Bf 109 fighters were a navalized version of the "E" model, designated as Bf 109T. Their wings were longer than the land-based model to allow for shorter take-off. The Ju 87s were to have been the "E" variant, which was a navalized version of the Ju 87D, and were modified for catapult launches and were equipped with arresting gear.
The ship was renamed Weser, but work was ceased in June 1943, before the conversion was completed. The unfinished vessel was then transferred to Königsberg, where she remained for the rest of the war. On 29 January 1945, the ship was scuttled before the advancing Soviet Red Army could seize her. The Soviet Navy nevertheless considered using the wreck for parts to complete the cruiser Lützow, a sister-ship of Seydlitz the Soviets had purchased before the outbreak of war. This was not carried out, however, and the ship was broken up for scrap.
Displacement: Design: 17,139 t (16,868 long tons; 18,893 short tons)
Length: 216 m (708 ft 8 in) overall
Draft: Full load: 6.65 m (21.8 ft)
Armament: 10 × 10.5 cm guns 10 × 3.7 cm guns 24 × 2 cm guns
Aircraft carried: 20 aircraft,