In 1940 and 1941, Demag and Hanomag worked on the development of new semi-tracked carrier that would replace Sd.Kfz.251. The new vehicle was a modified Sd.Kfz.251 with improved running gear, new Maybach HL 45 Z engine with 100hp and new transmission. The new Sd.Kfz.251 – HKp 602 / 603 remained in prototype form only. In 1941, Demag and Hanomag continued their development with HKp 605 and in 1942 with HKp 606. New vehicles featured simplified body, new Maybach HL 50 engine with 180hp and gearbox. Again, only prototypes were produced and entire project was stopped in favour of production of Sd.Kfz.251. The new simplified body design of HKp 606 was used in the development of Sd.Kfz.251 Ausf D body.