This paper reports on a proof-of-concept microelectromechanical system-based Fabry-Perot filter that is capable of electrically tuning within the long-wave 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 nanometer-scale as-released mirror flatness across an area of several hundred square micrometers 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%, full-width at half-maximum of spectral passband of approximately 500 nm, and out-of-band rejection greater than 40:1. Optical modeling 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 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 and 10.3-11.3 μm, respectively.