  The kinematic Sunyaev-Zel'dovich (kSZ) effect --- the Doppler boosting of
cosmic microwave background (CMB) photons due to Compton-scattering off free
electrons with non-zero bulk velocity --- probes the abundance and distribution
of baryons in the Universe. All kSZ measurements to date have explicitly
required spectroscopic redshifts. Here, we implement a novel estimator for the
kSZ -- large-scale structure cross-correlation based on projected fields: it
does not require redshift estimates for individual objects, allowing kSZ
measurements from large-scale imaging surveys. We apply this estimator to
cleaned CMB temperature maps constructed from Planck and Wilkinson Microwave
Anisotropy Probe data and a galaxy sample from the Wide-field Infrared Survey
Explorer (WISE). We measure the kSZ effect at 3.8-4.5$\sigma$ significance,
depending on the use of additional WISE galaxy bias constraints. We verify that
our measurements are robust to possible dust emission from the WISE galaxies.
Assuming the standard $\Lambda$CDM cosmology, we directly constrain $(
{f_{b}}/{0.158} ) ( {f_{\rm free}}/{1.0} ) = 1.48 \pm 0.19$ (statistical error
only) at redshift $z \approx 0.4$, where $f_{b}$ is the fraction of matter in
baryonic form and $f_{\rm free}$ is the free electron fraction. This is the
tightest kSZ-derived constraint reported to date on these parameters. The
consistency between the $f_{b}$ value found here and the values inferred from
analyses of the primordial CMB and Big Bang nucleosynthesis verifies that
baryons approximately trace the dark matter distribution down to $\sim$Mpc
scales. While our projected-field estimator is already competitive with other
kSZ approaches when applied to current datasets (because we are able to use the
full-sky WISE photometric survey), it will yield enormous signal-to-noise when
applied to upcoming high-resolution, multi-frequency CMB surveys.
