The present invention relates to an X-ray system and method for generating 3D image data. The X-ray system comprises a radiation detector, a movable X-ray tube assembly and a control unit, wherein in a working state of the system the radiation detector is detachably positioned in a spatially fixed manner and the X-ray tube assembly can be moved into relative positions relative to the radiation detector. Projection images recorded in the relative positions can be processed together with the coordinates of the relative positions using an iterative reconstruction technique or other image processing methods so as to obtain 3D X-ray images. The X-ray tube assembly is attached to a motor-driven movable stand arm and can be displaced using a control unit. The stand base of the movable stand can be positioned in a spatially fixed manner, so that it has a defined fixed position relative to the radiation detector.