  A search is presented for the associated production of a standard model Higgs
boson with a top quark-antiquark pair ($\mathrm{t\overline{t}}$H), in which the
Higgs boson decays into a b quark-antiquark pair, in proton-proton collisions
at a centre-of-mass energy $\sqrt{s} =$ 13 TeV. The data correspond to an
integrated luminosity of 35.9 fb$^{-1}$ recorded with the CMS detector at the
CERN LHC. Candidate $\mathrm{t\overline{t}}$H events are selected that contain
either one or two electrons or muons from the $\mathrm{t\overline{t}}$ decays
and are categorised according to the number of jets. Multivariate techniques
are employed to further classify the events and eventually discriminate between
signal and background. The results are characterised by an observed
$\mathrm{t\overline{t}}$H signal strength relative to the standard model cross
section, $\mu = \sigma$ / $\sigma_{\mathrm{SM}}$, under the assumption of a
Higgs boson mass of 125 GeV. A combined fit of multivariate discriminant
distributions in all categories results in an observed (expected) upper limit
on $\mu$ of 1.5 (0.9) at 95% confidence level, and a best fit value of 0.72
$\pm$ 0.24 (stat) $\pm$ 0.38 (syst), corresponding to an observed (expected)
signal significance of 1.6 (2.2) standard deviations above the background-only
hypothesis.
