A tool and method for bonding layers of a shell by the differential pressure bonding process. The tool and method includes a plurality of separable mandrel segments that combine to form a mandrel having a longitudinal axis, an outer surface, an upper end, and at least one substantially continuous inner surface. The inner surface has a substantially axisymmetric shape having complex curvature. In this embodiment, the tool further includes a retort configured to at least partially shroud the outer surface and upper end of the hollow body. The retort includes at least one vacuum port. The tool is configured to facilitate the compression of a plurality of layers of a multi-layer shell having complex curvature as the shell layers and an interdisposed bonding material are heated to an elevated bonding temperature.