The development of multimode amplifier research for broadband and mode switching applications presents significant new challenges, as it brings into mode design, manufacturing implementation and measurement issues that are not particularly significant in single-mode operation. In this paper we describe our design approach to these challenges, with emphasis on the mode switching implementation of a W-band dual-mode extended interaction amplifier. This device was designed to produce peak power of about 1 to 1.5 KW with a dual-mode spacing of about 900 MHz. The circuit will be driven by a 2 A, 15 kV electron beam. Early results demonstrate a promising dual-mode circuit with a typical ladder-type output cavity for ease of processing and measurement.