  Massive stars form on different scales ranging from large, dispersed OB
associations to compact, dense starburst clusters. The complex structure of
regions of massive star formation, and the involved short timescales provide a
challenge for our understanding of their birth and early evolution. As one of
the most massive and luminous star-forming region in our Galaxy, W49 is the
ideal place to study the formation of the most massive stars. By classifying
the massive young stars deeply embedded into the molecular cloud of W49, we aim
to investigate and trace the star formation history of this region. We analyse
near-infrared $K$-band spectroscopic observations of W49 from LBT/LUCI combined
with $JHK$ images obtained with NTT/SOFI and LBT/LUCI. Based on $JHK$-band
photometry and K-band spectroscopy the massive stars are placed in a
Hertzsprung Russell diagram. By comparison with evolutionary models, their age
and hence the star formation history of W49 can be investigated. Fourteen O
type stars as well as two young stellar objects (YSOs) are identified by our
spectroscopic survey. Eleven O-stars are main sequence stars with subtypes
ranging from O3 to O9.5, with masses ranging from $\sim 20$ $M_{\odot}$ to
$\sim 120$ $M_{\odot}$. Three of the O-stars show strong wind features, and are
considered to be Of-type supergiants with masses beyond 100 $M_{\odot}$ . The
two YSOs show CO emission, indicative for the presence of circumstellar disks
in the central region of the massive cluster. The age of the cluster is
estimated as $\sim1.5$ Myr, with star formation still ongoing in different
parts of the region. The ionising photons from the central massive stars have
not yet cleared the molecular cocoon surrounding the cluster. W49 is comparable
to extragalactic star-forming regions and provides us with an unique
possibility to study a starburst in detail.
