Cone-beam breast computed tomography (CBCT) would be a promising modality in screening and diagnosis of breast, providing complete 3-dimensional images with little painful compression of breast during the imaging compared to conventional mammography and tomosynthesis. To date, all CBCT systems including a commercial one by Koning have been utilizing a typical filament-based x-ray tube. However, the filament-based x-ray tube even in a grid type has strict limitation in time resolution, of longer than few milliseconds, with a limited dose rate to cause a large motion blur in CBCT projection images. Micro-calcifications of less than 1 mm in early breast cancer could be hardly distinguished by using conventional CBCT systems. We tried to solve this problem by adopting a fast digital x-ray tube based on carbon nanotube (CNT) field emitters. We, for the first time, developed a rotational anode x-ray tube with CNT emitters for advanced CBCTs. The x-ray tube consisted of CNT paste-emitters and a rotating anode made of W/Re target, and was fully vacuum-sealed with a glass envelope. Ultra-short x-ray pulses of less than sub-ms with a moderate high current of more than 200 mA and a focal spot of ~0.3 in nominal value was successfully obtained. We performed preliminary studies on CBCT imaging using the digital x-ray tube and achieved 300 projection images for 10 s, great reducing motion blurs in the images. It is expected that the CNT digital x-ray tube developed improves CBCT imaging greatly and then promotes CBCT modality in breast screening and diagnosis.