In the period 2000­2003, the JET Neutral Beam Injection (NBI) System has been upgraded with the design goal of delivering more than 25MW of Deuterium beam power into the JET plasma. The programme of the JET Octant 8 Neutral Injector Box (NIB) upgrade is, to date, the largest modification of the JET machine completed within the current EFDA-JET framework. It involved modifications of various components of the JET NBI systems including significant changes to: a) Positive Ion Neutral Injectors (PINIs); b) beamline components; c) high voltage power supplies and d) control, interlock and data acquisition systems. The JET NBI upgrade project was completed in November 2003, when the last two upgraded PINIs were successfully commissioned for injection into the JET plasma. Various technical achievements that were accomplished during the lifetime of this project are discussed in the paper.Unforeseen technical difficulties that prevented achievement of the design goal (25MW injected into JET plasma) are also discussed, together with undertaken improvements that will enable JET NBI system to deliver 25MW in 2005.