  We present the first detailed study of the large, ~30 pc diameter,
inner-Galaxy HII region W 39. Radio recombination line observations combined
with HI absorption spectra and Galactic rotation models show that the region
lies at V(LSR) = +65.4+/-0.5 km/s corresponding to a near kinematic distance of
4.5+/-0.2 kpc. Analysis of radio continuum emission shows that the HII region
is being powered by a cluster of OB stars with a combined hydrogen-ionizing
luminosity of log(Q) >=50, and that there are three compact HII regions located
on the periphery of W 39, each with log(Q)~48.5 (single O7 - O9 V star
equivalent). In the infrared, W 39 has a hierarchical bubble morphology, and is
a likely site of sequential star formation involving massive stars. Kinematic
models of the expansion of W 39 yield timescales of order Myr consistent with a
scenario where the formation of the smaller HII regions has been triggered by
the expansion of W 39. Using Spitzer GLIMPSE and MIPSGAL data we show that
star-formation activity is not distributed uniformly around the periphery of W
39 but is concentrated in two areas that include the compact HII regions as
well as a number of intermediate-mass Class I and Class II YSOs.
