In this paper, we consider the problem of finding the optimal control u(-) of the followingdistributed parameter system on a reflexive Banach space: ( dx(t))/(dt)=Ax(t) + b(t, u(t)), u(t)∈U, x(0)=x, so that x(t) may hit the target Q(t) of the same space in the shortest possible time. When U is a setin another Banach space, is bounded and closed but not necessarity convex, the closure of the reach-able region R(t) for the above system is proved to be a convex set.Furthermore the necessaryand sufficient conditions for the maximum principle and a computing formula for the optimal timeto hold are derived. The boundary control problem for parabolic systems is discussed for applica-tion.