  We report the results of spectrophotometric observations of the massive star
MN18 revealed via discovery of a bipolar nebula around it with the Spitzer
Space Telescope. Using the optical spectrum obtained with the Southern African
Large Telescope, we classify this star as B1 Ia. The evolved status of MN18 is
supported by the detection of nitrogen overabundance in the nebula, which
implies that it is composed of processed material ejected by the star. We
analysed the spectrum of MN18 by using the code CMFGEN, obtaining a stellar
effective temperature of \approx 21 kK. The star is highly reddened,
E(B-V)\approx 2 mag. Adopting an absolute visual magnitude of M_V=-6.8\pm0.5
(typical of B1 supergiants), MN18 has a luminosity of log L/Lsun \approx
5.42\pm0.30, a mass-loss rate of \approx (2.8-4.5)\times10^{-7} Msun/yr, and
resides at a distance of \approx 5.6^{+1.5} _{-1.2} kpc. We discuss the origin
of the nebula around MN18 and compare it with similar nebulae produced by other
blue supergiants in the Galaxy (Sher 25, HD 168625, [SBW2007] 1) and the Large
Magellanic Cloud (Sk-69 202). The nitrogen abundances in these nebulae imply
that blue supergiants can produce them from the main sequence stage up to the
pre-supernova stage. We also present a K-band spectrum of the candidate
luminous blue variable MN56 (encircled by a ring-like nebula) and report the
discovery of an OB star at \approx 17 arcsec from MN18. The possible membership
of MN18 and the OB star of the star cluster Lynga 3 is discussed.
