At Guilford Courthouse, the American army was arrayed in three defensive lines. The attacking British deployed on a wide front and charged the first line, which was composed chiefly of North Carolina militia.
According to a Virginian militiaman listening from the second line, the “close firing began near the centre… and soon spread along the line.”  This suggests that the 23rd and 71st regiments were advancing more quickly across the fields in the center of the British line than the 33rd and Bose regiments were advancing across the more difficult terrain on the left and right of the British line.
Descriptions of the details of this attack vary considerably from source to source. For example, militiaman James Martin recalled that on his front the first shot was fired when his company commander successfully picked off a British officer:
I was posted in the front Line with… Captain Forbes a brave and undaunted Fellow we were posted behind a Fence & I told the Men to sit down until the British who were advancing came near enough to shoot when they came in about 100 yards I saw [a] British officer with a drawn sword driving up his mans [sic, men] I asked Captain Forbes if he could take him down he said [he] could for [he] had a good Rifle and asked me if he should shoot then I told him let him [come to with]in 50 yards and then take him down which he did it was a Captain of the British Army 
But other accounts make no mention of isolated shots. Instead, they described a mass volley coming from the North Carolinians. The American commander, Major-General Nathanael Greene, stated that the North Carolinians opened fire when the British were 140 yards distant. Conversely, Sergeant Roger Lamb of the 23rd Regiment claimed the Americans held their fire until the British had closed to within murderously close range. He wrote:
…when [we] arrived within forty yards of the enemy's line, it was perceived that their whole force had their arms presented, and resting on a rail fence, the common partitions in America. They were taking aim with the nicest precision. 
However it happened, the Americans’ reception was deadly. The 23rd and 71st regiments respectively lost 29% and 26% of their men at Guilford Courthouse, and most of these casualties occurred in front of the rail fence. 
The British may have fired repeatedly during their advance through the center fields, and this fire (plus the threat of the bayonet) routed the North Carolinians.
Nathan Slade recalled that:
The enemy approached us and were according to the best of my belief within eighty to an hundred yards of us when they made their first fire—my recollection is that most of us stood firm until after the second [British] fire. On the third fire there were but few if any of us left to receive it—all or nearly all had broke and retreated in great disorder. 
This retreat was witnessed by men on the left and right ends of the American lines.
Lieutenant-Colonel Henry Lee commanded the flank corps on the left of the North Carolinians. There he saw, to his “infinite distress and mortification, the North Carolina militia took to flight”. 
James Collins was a North Carolina militiaman posted 200 yards to the right of the center fields. There “he saw the disgraceful retreat of that portion of the militia which was placed behind the fence”. 
Possibly this retreat set off a wave of panic among neighboring militia units. John Wadkins recalled that the “part of the line in which he was exchanged three or four fires” with the enemy. However, the men “became alarmed by report that the enemy was surrounding them – and fled”. 
The 71st Foot (Fraser's Highlanders) fires a volley at the North Carolina militia (here and below, click to enlarge).
Regiment von Bose is staggered by a volley.
North Carolinians of Brigadier-General John Butler's brigade flee from British bayonets.
The British break through the first defensive line.
1. Houston's account appears in William Henry Foote (1855). Sketches of Virginia....
3. Copies of the accounts by Greene, Lamb, and Lee (among others) can be found in this compendium.
4. Based on the appendixes in Lawrence E. Babits & Joshua B. Howard (2009) Long, obstinate, and bloody: The battle of Guilford Courthouse.