The 307 had the same vertical tail as the early B-17s. A 307 was lost
in the late 1930s during a demo flight for KLM. The cause was traced
back to rudder lock - at very low airspeeds and high rudder deflection,
the hinge moment of the rudder could reverse. As a result, the pilot
couldn't center the rudder. It was found in wind tunnel testing that
the addition of a dorsal fin would prevent this. The dorsal fin would
shed a vortex, preventing rudder lock. The 307s were retrofitted with a
dorsal fin and the B-17s got one too.

After the crash, KLM vowed to never buy a Boeing product. They finally
broke this vow when they bought 747s.