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We present time-of-arrival (TOA) measurements and timing models of 47millisecond pulsars (MSPs) observed from 2004 to 2017 at the AreciboObservatory and the Green Bank Telescope by the North American NanohertzObservatory for Gravitational Waves (NANOGrav). The observing cadence was threeto four weeks for most pulsars over most of this time span, with weeklyobservations of six sources. These data were collected for use in low-frequencygravitational wave searches and for other astrophysical purposes. We detail ourobservational methods and present a set of TOA measurements, based on"narrowband" analysis, in which many TOAs are calculated within narrowradio-frequency bands for data collected simultaneously across a widebandwidth. A separate set of "wideband" TOAs will be presented in a companionpaper. We detail a number of methodological changes compared to our previouswork which yield a cleaner and more uniformly processed data set. Our timingmodels include several new astrometric and binary pulsar measurements,including previously unpublished values for the parallaxes of PSRs J1832-0836and J2322+2057, the secular derivatives of the projected semi-major orbitalaxes of PSRs J0613-0200 and J2229+2643, and the first detection of the Shapirodelay in PSR J2145-0750. We report detectable levels of red noise in the timeseries for 14 pulsars. As a check on timing model reliability, we investigatethe stability of astrometric parameters across data sets of different lengths.We report flux density measurements for all pulsars observed. Searches forstochastic and continuous gravitational waves using these data will be subjectsof forthcoming publications.

The NANOGrav 12.5 yr Data Set: Observations and Narrowband Timing of 47 Millisecond Pulsars