The first N.C. 170 was one early designation for U.S. 220 through Montgomery and part of Richmond counties. The road got this number between 1928 and 1930; it had previously been part of N.C. 512. See Map #1. N.C. 170 was one of the first violations of the state's original numbering system, which would have required that 170 spur from the original N.C. 17. But 17 was in a different part of the state.  In 1933, the first N.C. 170 became part of the evanescent U.S. 411, which itself was renumbered to U.S. 220 a year later. Any year now it'll be totally taken off the map in favor of I-73/74.  North Carolina also had a U.S. 170 during the same time the first 170 existed. Both N.C. and U.S. 170 were eliminated during the 1933-34 great renumbering.

1. 1929 Automobile  Green Book map

The second 170 was born around 1936. It was signed as a new state highway over the southernmost portion of today's N.C. 34, from Elizabeth City (Pasquotank County, on the coast) south to the settlement of Weeksville. 

In 1939 or early '40, 170 was extended to the north. It was co-signed over that era's N.C. 30 through Elizabeth City. Further north, it was signed over today's N.C. 34 to Sligo in Currituck County. The seminal 30 eventually became part of U.S. 158; see Map #2. (Before 1940, 170 from Belcross all the way to Sligo was part of an earlier alignment of N.C. 30. The 30 shown running east of Belcross was a new designation that year as well. This area has had a lot of renumberings.) 

In 1940 or early '41, 170 reached its maximum length, when it was extended over that era's N.C. 34 (today's N.C. 168) north into Virginia. The continuation of the road in Virginia was renumbered to 170 at this time, so if you want you can consider this part of the 1940 Virginia renumbering. See Map #3. 

Both 170s -- N.C.'s and Virginia's -- were renumbered to 168 around 1957. 
 

2. 1940 General Drafting map

3. 1953 official state map

 

The first 171 ran from Lenoir west to Edgemont in Caldwell County over today's N.C. 90. It was a spur from the original N.C. 17 (U.S. 321). See the map at the top of the page. 171 was first designated in the late 1920s, and became part of N.C. 90 in 1934 or 1935. 

Today's 171 was born around 1936 (1935 maps show neither old nor new 171). It remains unchanged since then, except that the name of the settlement where 171 has its southern terminus used to be called Mineola. It's now called Old Ford.

One more thing: The route markers on 172 through the base are a tad different than they are in the rest of the state. The white diamond's corners are sharp, not rounded, and the black background surrounding the diamond is surrounded by a white border. In fact these markers look better than what the state normally uses. 

JVincent asks: Is 172 ever closed during intensive military operations such as Operation Desert Storm? 

Every 17x highway in North Carolina, including the two U.S. routes, enters the state at its southern border (coming up from S.C. or Georgia) -- except for this one.

The first 175 was located in the Grandfather Mountain area. It spurred from the original N.C. 17 in Blowing Rock, running west along today's U.S. 221 to Linville, then further west over today's N.C. 181 to Newland. It's shown in the map at the top of the page. 
This designation lasted from the early 1920s to around 1929, when it was replaced by extensions of 181 (Newland-Linville) and N.C. 691 (Linville to B-Rock). 
The stretch of today's 221 between Linville and Blowing Rock (the real twisty part) has had a lot of different numbers: After it was 175 and 691 it also spent a year or two as N.C. 28 before the state settled on the U.S. designation for good in 1933. 

Today's 175 got its number around 1967; until then the road had gone unnumbered.

By 1930, 176 was renumbered as an extension of N.C. 691. This was also the fate of the original N.C. 175.

None of the four existing 170-179 state routes crosses a county line. In fact U.S. 178 doesn't, either, and many of the dead 17x's were similarly short. North Carolina's 17x highways have always been wimpy, wimpy, wimpy.