The spiral model is based on the concepts of evolutionary models. The Evolutionary Model, like the Incremental Model, develops the system in multiple builds, but differs from the Incremental Model in acknowledging that the user needs or some other aspects affecting the generation of system requirements are not fully understood, and that all requirements cannot be defined early in the development cycle.
The evolutionary models are often used in high risk projects, such as pure research and development, projects that depend on unproven technology and projects in which failure could have major impact on the company’s business.
The main difference between the evolutionary models and the sequential and incremental models is that in evolutionary models all the requirements of the final system or product are not known when the development starts. Initially the requirements are partly defined and then refined and extended with each successive (internal) release, till a satisfactory solution is reached.
The Spiral model, defined by Barry Boehm, describes a model where a prototype is refined to a product or system in a series of cycles. Using the spiral model software is developed in a series of incremental releases. During early iterations, the incremental release might be a paper model or prototype. The Spiral Model adds risk analysis and prototyping activities at the beginning of each phase defined in the Waterfall Model to address the perceived risk areas of a project.
The Spiral Model focuses on significant areas of risk, studies alternative solutions, and develops prototypes and simulations to explore the strengths and weaknesses of candidate solutions.
The prototype may very well be thrown away after each cycle, and a new prototype is developed until the final loop in which the final product or system is created. That is the Spiral model is not necessarily incremental.
There are many modifications of original spiral model available. The original spiral model (Boehm, 1988) was first modified by Software Productivity Consortium in 1994 and then later modified by Boehm himself, as “win-win spiral model” in 1996.