The NK-31 and NK-39 engines were designed during the height of the US / Soviet Space Race to place a man on the moon.  Built by the renowned gas turbine design bureau, ND Kuznetsov Joint Stock Company Scientific-Technical Complex of Samara Russia, the NK-31 and NK-39 engines are upgraded versions of the NK-19 and NK-21 engines which flew on the first four flights of the Russian N-1 moon launcher.  The NK-31 and NK-39 engines were designed to provide improved thrust and reliability. Despite being developed in the late 1960's, the NK family is still the highest performing LOX / RP set of rocket engines ever produced. 

Today, the NK-31 and NK-39 engines are being marketed commercially by GenCorp Aerojet of Sacramento, CA.  The NK-39, which powered the Russian N-1 third stage, does not gimal or restart.  A single NK-31 powered the N-1 fourth stage, the Block-G (N-1 Translunar Injection Stage), and is capable of gimballing and restarting.  The NK-31 was proposed by Aerojet for the X-34 as a back-up to the NASA Fast Trak engine.  To date, there are no commercial customers.