  Star formation primarily occurs in filaments where magnetic fields are
expected to be dynamically important. The largest and densest filaments trace
spiral structure within galaxies. Over a dozen of these dense
($\sim$10$^4$\,cm$^{-3}$) and long ($>$10\,pc) filaments have been found within
the Milky Way, and they are often referred to as "bones." Until now, none of
these bones have had their magnetic field resolved and mapped in their
entirety. We introduce the SOFIA legacy project FIELDMAPS which has begun
mapping $\sim$10 of these Milky Way bones using the HAWC+ instrument at
214\,$\mu$m and 18$\farcs$2 resolution. Here we present a first result from
this survey on the $\sim$60\,pc long bone G47. Contrary to some studies of
dense filaments in the Galactic plane, we find that the magnetic field is often
not perpendicular to the spine (i.e., the center-line of the bone). Fields tend
to be perpendicular in the densest areas of active star formation and more
parallel or random in other areas. The average field is neither parallel or
perpendicular to the Galactic plane nor the bone. The magnetic field strengths
along the spine typically vary from $\sim$20 to $\sim$100\,$\mu$G. Magnetic
fields tend to be strong enough to suppress collapse along much of the bone,
but for areas that are most active in star formation, the fields are notably
less able to resist gravitational collapse.
