Although called Spähkreuzer - Reconnaissance Cruiser - those ships were basically large destroyers capable for Atlantic Operations. Based on traditional destroyer design, several project studies started in 1938 to develop a large destroyer of the size of a CL. Like most German CLs, those ships were planed to get a mixed propulsion system, geared turbines for high speed and diesel engines for long cruises.
It was planed to use this ships in combinations of the new battleships of the Z-Plan in the North Atlantic where the Spähkreuzer should be the eyes of a larger battle fleet.
With the lessons learned by the first naval actions in World War II, the plans for the Spähkreuzer were modified several times, after the "Spähkreuzer 38" later designs were called "Spähkreuzer 39" and "Spähkreuzer 40" which had improved range, armor protection and a float plane on board.
The first three ships (SP1, SP2, and SP3, formerly named Z40, Z41, Z42) were ordered in February 1941 at the Germania Shipyard in Kiel. In December of 1941, the engines for the next three ships (SP4, SP5, SP6) were ordered too, but not the ships themselves.
Spähkreuzer SP1 - SP3
Only SP1 was laid down, but its construction planes were destroyed during a air raid. In April 1942, construction of all Spähkreuzer was stopped and the focus switched to the construction of normal destroyers.