Battlecruisers
Although interest in the P class ships and the battlecruiser proposal waned for a time in late 1937 and early 1938, it was quickly revived on 28 April 1938 with Hitler's public airing of his views on the 1935 Anglo-German Treaty and the consequent increased possibility of war with the United Kingdom. Calling the Naval Staff and Admiral Erich Raeder to him, Hitler laid out his thoughts of a strong navy that could pose enough danger to the British such that they would enter into an alliance with Germany rather than go to war. As decided here, Plan Z entailed two task forces each centered around three H class battleships and one aircraft carrier, with cruisers and destroyers as escorts. In war, these forces would collaborate with the three battlecruisers by occupying convoy screens while U-boats and one or more of the O class took out the cargo-carrying merchant ships.
As part of the plan, design work on the P class was halted in mid-1939 in favor of the O class. Displacement was limited to 30,000 tonnes (30,000 long tons) in the new designs so that the length of construction would be shortened from the normal four or more years that a battleship required, to an estimated three to three and a half years. Required characteristics for the battlecruisers was a displacement of 30,000 tonnes (30,000 long tons), a main battery of six 380 mm (15 in) guns, a secondary battery of dual purpose guns, a top speed of 34 kn (39 mph; 63 km/h), a range of 15,000 mi (24,000 km) at 19 kn (22 mph; 35 km/h), and enough armor to counter the 203 mm (8.0 in) guns of heavy cruisers.
Like the P class, there were problems with fitting an all-diesel drive into the hull of the ships. It was enough that the design team decided that it would not be prudent to use an all-diesel arrangement; instead, a hybrid diesel-steam turbine propulsion plant was to be used. The change allowed the central armored citadel to be reduced by 9 m (30 ft) and the aft beam to be lowered by 3.5 m (11 ft).
Although plans were not finalized, the third battlecruiser, "Q", was ordered from Germaniawerft in Kiel on 8 August 1939. One month after, the contract design was refined. The displacement was increased to 31,652 tonnes (31,152 long tons) (design) and 35,945 t (35,377 long tons) (full-load), while the draft at those two figures was 8 ft (2.4 m) and 8.8 m (29 ft), respectively. At the waterline, the length was set at 246 m (807 ft) and beam at 30 m (98 ft). The main battery was the same as previously (six 380 mm (15 in)/47 caliber guns in dual turrets), while the secondary battery was split between six paired 150 mm (5.9 in)/48 caliber anti-surface guns and paired 105 mm (4.1 in)/65 caliber anti-aircraft guns because German designers had not been able to develop a satisfactory dual-purpose gun. Light anti-aircraft guns were eight 37 mm in four dual mounts and twenty 20 mm autocannon in single mounts. Twelve 533 mm (21.0 in) torpedo tubes finished out the armament aboard the ships. Four floatplanes for scouting were planned. At the design displacement, the maximum speed was reduced by half a knot, to 33.5 kn (38.6 mph; 62.0 km/h); this required a top shaft horsepower of 173,600.
By 1940, project drawings for the three battlecruisers were complete. They were reviewed by both Hitler and Admiral Raeder, both of whom approved. However, outside of "initial procurement of materials and the issuance of some procurement orders", the ships' keels were never laid.
Read more about this topic: O Class Battlecruiser