We present a mechanized theory of program refinement that allows for the stepwise development of imperative programs in the Coq proof assistant. We formalize a design language with support for gradual refinement and a calculus which enforces correctness-by-construction. A notion of program design captures the hierarchy of refinement steps resulting from a development. The underlying theory follows the predicative programming paradigm where programs and specifications are both easily expressed as predicates, which fit naturally in the dependent type theory of the proof assistant.