In this paper we propose a novel benchmark for numerical solutions of transient and ultrawideband scattering from perfectly conducting targets. The target for the problem comprises two open concentric spherical shells with a common axis of symmetry, which represents the direction of propagation of the incident field. This geometry includes a variety of physical features including surfaces, edges, cavities and an inclusion within a cavity. It provides a range of scattering mechanisms, including multiple body interactions and cavity resonance effects, the relative strength of which can be varied via the parameters of the problem. The time harmonic scattering problem is solved via the method of regularization providing a semi-analytic solution of arbitrary predetermined accuracy with good stability and convergence properties. Solution of this time harmonic problem provides a benchmark for both time domain and frequency domain numerical solutions of Maxwell's equations. An example is given of this benchmark problem, as applied to a time domain integral equation method.