The US 15 corridor from Williamsport to Painted Post is part of the I-99 corridor (along with US 220 from Bedford to Williamsport). At the time I-99 was envisioned, US 15 consisted of a mix of freeway segments and two-lane roads; over time, Pennsylvania and New York worked to fill in the gaps, with the New York portion finished and the I-99 designation added in 2014.
The images below have been obtained from the U.S. Geological Survey (USGS) Eros Archive and Google Earth.
The above image (Google) shows the completed freeway from the Pennsylvania state line to the existing freeway in Presho.
Pennsylvania filled in its portion of the freeway gap between Tioga and Presho first, and for a few years, there was a temporary end at Watson Creek Road. The images above are from 2003 (top, Google/USDA/FPAC/GEO), 2013 (middle, Google), and 2021 (bottom, Google).
The freeway from Presho to NY 17 originally connected to the old alignment of US 15. The images above are from 2005 (top, Google/USDA/FPAC/GEO) and 2016 (bottom, Google).
The original interchange between US 15 and NY 17 was a traffic circle. At the time, the Corning Bypass on NY 17 hadn't been completed either, and NY 17 went through downtown corning on modern NY 352. The US 15 freeway from Presho to Painted Post had at-grade intersections with the ramps from NY 17 (and later I-86). The modern interchange between US 15 (later I-99) and I-86 was completed in the 2000s. The images above are from 1968 (top, USGS), 2002 (middle, Google/U.S. Geological Survey/New York GIS), and 2016 (bottom, Google).