Mechta,
A very short
précis.
The RAF E3D's weren't strictly off the shelf as the E3 production line had been shut down years before. The build line wasn't there. However, Boeing had the facility and the plans. The RAF team knew exactly what they wanted, and how much the total budget allowance was. One of the slightly bizarre issues covered the American favoured boom refuelling system. The RAF didn't want this, just probe and drove, but to design the boom system out would have cost circa $500,000 per airframe. The E3D's have both refuelling systems. Saved money and increased capability. The only notable carry over from the AEW3 was the one good piece of kit the RAF liked, and the Loreal pods were removed from Nimrod and installed on to the E3D's at production. Utilising this piece of proven kit gave further financial savings.
First-hand knowledge of mission requirements along with experience on the NATO E3 fleet helped this programme run smoothly from inception to delivery without any of the 'normal' delays and cost overruns we have become used to.
There are others on here who could help with deeper explanations on how well executed this whole acquisition project was accomplished without crossing the OPSEC boundary.
Camlobe