Field-emission x-ray source arrays have been studied for both tomosynthesis and CT applications, however these arrays tend to have limited output. We propose the use of multi-source x-ray arrays using thermionic cathodes, contained within a single vacuum housing. A prototype 3-source x-ray array has been fabricated and tested, and the utility of multi-x-ray-source arrays has been demonstrated using physical simulations in both tomosynthesis and in cone beam CT. The prototype x-ray tube made use of a cylindrical molybdenum anode, machined to have 3 specific focal tracks. Grid-controlled cathode assemblies were fabricated and aligned to each focal tract, and the individual x-ray focal spots were evaluated with a star pattern at 35 kV and 40 mA. The 3-source assembly was used to physically simulate tomosynthesis imaging geometry, and tomosynthesis images of a lemon were obtained. Physical simulations using a cone beam breast CT scanner were also performed, by vertically moving the single x-ray source into 5 different locations - simulating 5 different source positions. A new geometry for cone beam CT imaging is proposed, where each source of a multi-x-ray source array is individually collimated to eliminate rays involving large cone angles. This geometry also allows three sources to be simultaneously pulsed onto a single flat panel detector, achieving better duty cycle and view sampling in cone beam CT. A reconstruction algorithm was written to accommodate the different source positions, and phantoms designed to demonstrate cone beam artifacts were imaged. The tomosynthesis images illustrate appropriate depth resolution in the test object. Analysis of the CT data demonstrate marked improvement compared to one source. We conclude that multi-source x-ray arrays using thermionic cathodes will have important applications in medical imaging, especially breast tomosynthesis and cone beam computed tomography.