Definitely not my area of expertise, but the subject does come up in North Korean histories -- mostly because it presents a propaganda problem for the Kims!

It's surprisingly tough to get reliable information about Pyongyang's history, at least in English. North Korean state archaeologists and historians find themselves unemployed at best when they reach conclusions that the Kim regime doesn't particularly like. This is a toughie for them, because Pyongyang's had relevance as a trading, military, and diplomatic outpost for centuries, but in contrast to Seoul, it's never served as the capital of a unified Korean state. Interestingly, if Kim il-Sung had won the Korean War, Pyongyang wouldn't have been his pick as a capital, but more on this in a moment.

Pyongyang was the more important of the two cities during the Three Kingdoms period between 427 to 668 CE when it became the capital of Koguryeo, the largest of the three Korean states on the peninsula. As such, North Korean propaganda focuses almost exclusively on Koguryeo when it discusses this era; Silla and Baekje go begging for coverage, even though it was a combination of Silla and Tang China that forcibly unified the peninsula.

Seoul existed as a city at that point, but it didn't really start having significant political or economic relevance until it became the capital of Joseon two years after the beginning of the dynasty in 1394. It would remain the capital of the Korean state for the next 500+ years until 1897. It then became the capital of the short-lived Korean Empire, and was subsequently picked as the colonial capital by the Japanese after they formally annexed Korea in 1910 (mostly because it was centrally-located, the most technologically advanced city on the peninsula, and had the greatest number of foreign trade links).

Andrei Lankov in North of the DMZ notes that Pyongyang's status as the North Korean capital is actually owed to a Soviet general's pragmatism. When the Soviets rolled down onto the peninsula in August 1945, Ivan Chistiakov (the general overseeing the occupation of the northern half of the peninsula) was asked whether he wanted his headquarters in Hamhung (then as now, North Korea's second-largest city and a major industrial center) or Pyongyang. He picked the latter solely because Pyongyang could be more easily reached by a mechanized army (it's located on a river plain, as opposed to Hamhung, which is mostly surrounded by mountains) and was a bit more conveniently located for administrative purposes. When the Soviets passed off government responsibilities to the Koreans, Kim and his brethren inherited the Soviet Civil Administration's offices in Pyongyang.

And while the regime isn't eager to advertise this, the first North Korean constitution, which was written in 1948 under the assumption that the peninsula would reunify within a few years, declares its capital as Seoul. From Kim's perspective, the city was being occupied by an illegitimate government but would soon be restored to its rightful place as the center of a unified Korean state, presumably under his guidance.

That didn't happen. Unsurprisingly, North Korea's archaeologists and historians were subsequently told in no uncertain terms to go "find" evidence of Pyongyang's historic brilliance and importance.

Edit: Fixed a word.