Deutsche Bahn doesn't manufacture rolling stock. They buy it from Siemens, Stadler, Talgo, Alstom etc...

Edit: AFAIK, some of it - mainly high-speed trains - is designed according to DB specs and subsequently offered under a new name (and with changes) to other train companies. For example DB ICE 3 (manufactured by Siemens) / Siemens Velaro.