  We have recently measured the angle between the spin and orbital angular
momenta of PSR B1534+12 to be either 25+/-4 deg or 155+/-4 deg. This
misalignment was almost certainly caused by an asymmetry in the supernova
explosion that formed its companion neutron star. Here we combine the
misalignment measurement with measurements of the pulsar and companion masses,
the orbital elements, proper motion, and interstellar scintillation. We show
that the orbit of the binary in the Galaxy is inconsistent with a velocity kick
large enough to produce a nearly antialigned spin axis, so the true
misalignment must be ~25 deg. Similar arguments lead to bounds on the mass of
the companion star immediately before its supernova: 3+/-1 Msun. The result is
a coherent scenario for the formation of the observed binary. After the first
supernova explosion, the neutron star that would eventually become the observed
pulsar was in a Be/X-ray type binary system with a companion of at least 10--12
Msun. During hydrogen (or possibly helium) shell burning, mass transfer
occurred in a common envelope phase, leaving the neutron star in a roughly
half-day orbit with a helium star with mass above ~3.3 Msun. A second phase of
mass transfer was then initiated by Roche lobe overflow during shell helium
burning, further reducing both the helium star mass and orbital period before
the second supernova. Scenarios that avoid Roche lobe overflow by the helium
star require larger helium star masses and predict space velocities
inconsistent with our measurements. The companion neutron star experienced a
velocity kick of 230+/-60 km/s at birth, leading to a systemic kick to the
binary of 180+/-60 km/s.
