The JET Neutral Beam (NB) heating system is being upgraded as a part of the ongoing JET Enhancement Programme. This is one of the largest upgrades of the JET machine carried out within the EFDA-JET framework. The main goals of the project are to increase the NB power delivered to JET plasma, to increase the beam pulse duration and to improve the availability and reliability of the JET NB system. The upgrade of the system is being carried out through the modification of the two existing Neutral Injector Boxes (NIBs), each equipped with up to eight Positive Ion Neutral Injectors (PINIs). Significant changes of the JET NB system will be carried out within the next few years and will include modification of all PINIs, modification or replacement of various beamline components and corresponding instrumentation, procurement and installation of new High Voltage Power Supply (HVPS) units and corresponding control systems and refurbishment of the 36kV power distribution. Various physics, engineering and planning issues related to this project, as well as the current status of the project are discussed in detail. Particular attention is given to the results of a PINI prototype test, which are of crucial importance for the successful completion of the entire Enhancement Programme. Upon the completion of the project in 2009, JET NB system should be capable of delivering more than 34MW of deuterium beam power into the JET plasma for a duration of up to 20 seconds with improved reliability. This will significantly enhance overall capabilities of the JET machine in support of ITER development.