Spiral model

The spiral model is a software development process combining elements of both design and prototyping-in-stages, in an effort to combine advantages of top-down and bottom-up concepts.

The spiral model was defined by Barry Boehm in his article A Spiral Model of Software Development and Enhancement from . This model was not the first model to discuss iterative development, but it was the first model to explain why the iteration matters. As originally envisioned, the iterations were typically months to years long.

Each phase starts with a design goal and ends with the client (who may be internal) reviewing the progress thus far. Analysis and engineering efforts are applied at each phase of the project, with an eye toward the end goal of the project.

The Spiral model is used most often in large projects (by companies such as IBM and Microsoft) and needs constant review to stay on target. For smaller projects, the concept of agile software development is becoming a viable alternative. The US military has adopted the spiral model for its Future Combat Systems program.

Estimates (i.e. budget, schedule, etc.) get more realistic as work progresses, because important issues are discovered earlier.
It is more able to cope with the (nearly inevitable) changes that software development generally entails.
Software engineers (who can get restless with protracted design processes) can get their hands in and start working on a project earlier.

From Wikipedia