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Here we present the second results of SOFIA-FORCAST mid-infrared (MIR) imaging survey toward the Galactic giant HII (GHII) regions. The second target we performed in-depth study is M17 that is the closest GHII region from Sun. M17 has been studied for more than 250 years but the mysteries of its formation mechanisms associated with a massive stellar cluster at the center, NGC6618, only recently start to be revealed. In this study, we utilize the FORCAST 20 and 37μm images of M17 that are non-saturated high angular resolution observation for the first time in this wavelength regime. In conjunction with Spitzer and Herschel MIR to FIR images, we construct the spectral energy distributions (SEDs) of the individual young stellar objects (YSOs) and proto/young star clusters defined by FORCAST images as point-like and extended sources, respectively. As the synergy with a theoretical model of massive star formation developed by Zhang et al. (2011), we could retrieve the fundamental properties of the point-like sources and define the seven out of sixteen sources as the massive YSO candidates. The global star formation history of M17 has been examined by comparing two different evolutionary tracers of proto/young star clusters in different regions of M17, the luminosity-to-mass ration and virial parameter, by investigating additional sub-mm (JCMT) and 13CΟ(1-0) (Nobeyama 45m) data. The results show the northern bar of M17 is relatively older than southern bar and other locations have different evolutionary stages indicating the entire M17 cloud is not coeval. We also show that the potential high contamination of 20μm images by [SIII] line emission from the ionized regions.