This paper reports on a proof-of-concept MEMSbased Fabry-Perot filter that is capable of electrically tuning within the longwave infrared thermal imaging band of 8-12 µm. The device employs a single-layer quarter-wavelength thick tensile germanium membrane for the suspended top mirror in order to achieve nanometre-scale as-released mirror flatness across an area of several hundred square microns without any extraneous stress management techniques. Mechanical and optical characterization of the tunable filters of various sizes are presented and compared. A 200 µm dimension square filter is demonstrated with <100 nm top mirror bowing and near-theoretical spectral characteristics across the entire tuning range of 8.5-11.5 µm; namely, peak transmission above 80%, FWHM of spectral passband of approximately 500 nm, and out-of-band rejection greater than 40:1. Optical modelling shows that this filter can achieve a pixel-to-pixel transmission peak wavelength variation of less than 1.2% across the entire 200 µm × 200 µm optical imaging area. These results exceed the optical performance requirements for passive multispectral thermal imaging applications based on large-area focal plane arrays. In comparison, the 500-µm and 1000-µm dimension filters are shown to exhibit significant mirror bowing with actuation and, thus, for a pixel-to-pixel transmission peak wavelength non-uniformity of <4%, demonstrate narrower usable spectral tuning ranges of 9.3-11.4 µm and 10.3-11.3 µm, respectively.