For the required upgrades of the Electron Cyclotron Resonance Heating system at the stellarator Wendelstein 7-X, the development of a 1.5 MW 140 GHz Continuous Wave (CW) prototype gyrotron has started. KIT has been responsible to deliver the scientific design of the tube (i.e. the electron optics design and the RF design), with contributions from NKUA and IPP. The prototype gyrotron has been ordered at the industrial partner, Thales, France, and is expected to be delivered in 2021. In parallel, a short-pulse pre-prototype gyrotron has been developed at KIT, to provide the means for a first experimental validation of the scientific design in ms pulses, prior to the construction of the CW prototype. This paper reports on the status of the 1.5 MW CW gyrotron development, focusing on the scientific design and its numerical and experimental validation.