With the current systems engineering approach to development, pretty much all of the design is done before you start building. There is a nice graph that I cant find about now, but will keep looking, and just describe it here. On the X axis you have time, and on the Y, money. You have two lines, cost committed, and cost spent. When you are doing design work, you employ a large number of people, but they are utilising relatively little resources, and almost never use much resources. On the other hand, when you finish more and more pieces of the design, the cost you have committed in the development increases. Therefore, the low financial burden of cost spent on design gives a low gradient, but the cost committed starts off very steep. Approaching half way you're building test pieces, building plants etc, the money spent begin to climb, whilst the design work is winding down, so the committed cost line climbs less and less rapidly. Usually about 60% of the way through the process, you start building an aeroplane. At this point, the spent cost reaches its highest gradient, and pretty much stays there until project end, and the design is wound down. the lines end converging at a point of the final time of the development process, and the total money spent. Both lines have positive gradients along their entire length, as you are always spending money, and committing more.
Hope that's clear, I will try to answer questions, and find the graph tomorrow.