During the earlies part of our nations history, slavery actually didn't exist. It wasn't until the mid 1700's that laws and customs developed which turned servitude into racially segregated slavery.
In the early American colonies there were no laws regarding slavery. When the first slaves arrived in Jamestown the colonists had no idea what their legal status even was, so they turned them into indentured servants and gave them the same legal protection as white servants. For a time, black and white women worked side by side in the fields and both races enjoyed the same legal status. A slave who converted to Christianity was freed since English law prohibited the enslavement of Christians. Early census's of the colonies recorded black slaves as "servants" and there wasn't a single person in the colonies that used "white" or "black" as social dividers. You were "Englishmen" or "servant" or "christian" or "artisan" etc.
Eventually as the colonies grew, slavery began to become a much more attractive labor solution. Servants were eventually freed and needed to be replaced. Servants were also protected by the law and could sue their employers in open court. Also, once free, they were given land which threatened the landed elite of the colonies. Having a stable, life-long source of labor that was recognizable by visible differences in skin color became an obvious solution.
Although other colonies actually recognized slavery earlier (Massachusetts legally recognized slavery in 1641) Virginia declared was the first to declare slaves were property. A 1705 law stated that "All servants imported and brought in this County... who were not Christians in their Native Country... shall be slaves. A Negro, mulatto and Indian slaves ... shall be held to be real estate."
Since Africans were already believed to be an inferior race in the eyes of most European countries, it wasn't much of a leap to give them legally inferior rights. Most slave owners sought solice in the Bible, which they said condoned slavery and supported the importation of Africans.
With both religion and the law on their side, England soon became the largest importer of slaves in the world, surpassing both Spain and Portugal by the 1698.