The Windows Development Timeline

Historically, Microsoft has taken approximately 40 months to design, develop, and test a major release of Windows. Shown above are the development periods for several versions of Windows. The development period of a version runs from the release to manufacturing (RTM) of one version to the RTM of the next. For example, Windows 2000 development started in late 1995 and the product shipped 44 months later, marking the start of the Windows XP and .NET Server cycle. Windows .NET Server, another major release, has taken 38 months (based on the estimated ship date at the end of 2002). Windows XP was originally intended to be released with Windows .NET Server, but after 22 months of development, Microsoft decided to release it as a client-only update.

Estimating dates for service packs is more difficult, because they are driven by the number of product and security bugs Microsoft determines it has to fix. But it is likely that there will be at least one service pack per supported version of Windows per year. These service packs also impact development time for major releases of Windows, as would any further coding halts, such as the two-month code review in early 2002 to implement more security-conscious development practices.