While I am totally committed to the accuracy of the KJV, it has puzzled me as to why that was not rendered as "heavens" (plural) even though the Hebrew is clearly plural, and theologically it also makes perfect sense. The Septuagint also corresponds to the KJV. I don't believe we have seen a good explanation for this anomaly.
haš·šā·ma·yim
הַשָּׁמַ֖יִם
the heavens
haš·šā·ma·yim
הַשָּׁמַ֖יִם
the heavens
הַשָּׁמַ֖יִם. as the heavens is true as it rendered in the other passages of the Bible but it would be also be truer using the same Hebrew text who we have evidently used the word in singular. If we look on the NASB word usage alone, that could not be an even! Assuming it is an even, and then the treatment is found in the context. Context determines the true rendering otherwise the Bible is full of contradiction but who will go with that?
Of course, I have already cited non bias sources including Strong. Here are the reasons why it must be heaven and not heavens in the particular text of Genesis 1:1
1. Because if it’s been rendered heavens is true at the beginning of creation then God would not have need of further creation of vast expanse of firmament/(use as plural meaning heavens) universe, galaxies or Milky ways which created until day 2.
2. According to William Rosenau, (google it) that such word in Hebrew, however, the plural form may identify size rather than number in certain contexts. Such a plural is called a "plural of extension or amplification"
3. Further, Gessenius, a critical scholar give us an example of the plural extension to some Hebraic expressions. Examples of (a): Plurals of local extension to denote localities in general, but especially level surfaces (the surface-plural), since in them the idea of a whole composed of innumerable separate parts or points is most evident, as שָׁמַ֫יִם(§88d) heaven (cf. also מְרוֹמִים heights of heaven,
https://en.wikisource.org/wiki/Gesenius'_Hebrew_Grammar/124._The_Various_Uses_of_the_Plural-form
So simple and short, the KJV is correct.
God bless