  Here we report on the first successful exoplanet transit observation with the
Stratospheric Observatory for Infrared Astronomy (SOFIA). We observed a single
transit of the hot Jupiter HD 189733 b, obtaining two simultaneous primary
transit lightcurves in the B and z' bands as a demonstration of SOFIA's
capability to perform absolute transit photometry. We present a detailed
description of our data reduction, in particular the correlation of photometric
systematics with various in-flight parameters unique to the airborne observing
environment. The derived transit depths at B and z' wavelengths confirm a
previously reported slope in the optical transmission spectrum of HD 189733 b.
Our results give new insights to the current discussion about the source of
this Rayleigh scattering in the upper atmosphere and the question of fixed limb
darkening coefficients in fitting routines.
