The emission nebula M17 contains a young ~1 Myr-old open cluster; the winds from the OB stars of this cluster have blown a superbubble around the cluster. ROSAT observations of M17 detected diffuse X-ray emission peaking at the cluster and filling the superbubble interior. The young age of the cluster suggests that no supernovae have yet occurred in M17; therefore, it provides a rare opportunity to study hot gas energized solely by shocked stellar winds in a quiescent superbubble. We have analyzed the diffuse X-ray emission from M17, and compared the observed X-ray luminosity of ~2.5*10^33 ergs/s and the hot gas temperature of ~8.5*10^6 K and mass of ~1 M_Sun to model predictions. We find that bubble models with heat conduction overpredict the X-ray luminosity by two orders of magnitude; the strong magnetic fields in M17, as measured from HI Zeeman observations, have most likely inhibited heat conduction and associated mass evaporation. Bubble models without heat conduction can explain the X-ray properties of M17, but only if cold nebular gas can be dynamically mixed into the hot bubble interior and the stellar winds are clumpy with mass-loss rates reduced by a factor of >=3. Future models of the M17 superbubble must take into account the large-scale density gradient, small-scale clumpiness, and strong magnetic field in the ambient interstellar medium. ; Comment: 21 pages, 4 figures, to be published in the Astrophysical Journal, June 2003