There are many, many
evolving regional British and American accents, so the terms “British
accent” and “American accent” are gross oversimplifications. What a lot
of Americans think of as the typical "British accent” is what's called
standardized Received Pronunciation (RP), also known as Public School
English or BBC English. What most people think of as an "American
accent," or most Americans think of as "no accent," is the General
American (GenAm) accent, sometimes called a "newscaster accent" or
"Network English." Because this is a blog post and not a book, we'll
focus on these two general sounds for now and leave the regional accents
for another time.
English colonists established their first permanent settlement in the
New World at Jamestown, Virginia, in 1607, sounding very much like
their countrymen back home. By the time we had recordings of both
Americans and Brits some three centuries later (the first audio
recording of a human voice was made in 1860),
the sounds of English as spoken in the Old World and New World were
very different. We're looking at a silent gap of some 300 years, so we
can't say exactly when Americans first started to sound noticeably different from the British.
As for the "why," though, one big factor in the divergence of the accents is rhotacism. The General American accent is rhotic and speakers pronounce the r in words such as hard. The BBC-type British accent is non-rhotic, and speakers don't pronounce the r, leaving hard sounding more like hahd.
Before and during the American Revolution, the English, both in England
and in the colonies, mostly spoke with a rhotic accent. We don't know
much more about said accent, though. Various claims about the accents of
the Appalachian Mountains, the Outer Banks, the Tidewater region and
Virginia's Tangier Island sounding like an uncorrupted Elizabethan-era English accent have been busted as myths by linguists.
Talk This Way
Around the turn of the 18th
19th century, not long after the revolution, non-rhotic speech took off
in southern England, especially among the upper and upper-middle
classes. It was a signifier of class and status. This posh accent was
standardized as Received Pronunciation and taught widely by
pronunciation tutors to people who wanted to learn to speak fashionably.
Because the Received Pronunciation accent was regionally "neutral" and
easy to understand, it spread across England and the empire through the
armed forces, the civil service and, later, the BBC.
Across the pond, many former colonists also adopted and imitated
Received Pronunciation to show off their status. This happened
especially in the port cities that still had close trading ties with
England — Boston, Richmond, Charleston, and Savannah. From the
Southeastern coast, the RP sound spread through much of the South along
with plantation culture and wealth.
After industrialization and the Civil War and well into the 20th
century, political and economic power largely passed from the port
cities and cotton regions to the manufacturing hubs of the Mid Atlantic
and Midwest — New York, Philadelphia, Pittsburgh, Cleveland, Chicago,
Detroit, etc. The British elite had much less cultural and linguistic
influence in these places, which were mostly populated by
the Scots-Irish and other settlers from Northern Britain, and rhotic
English was still spoken there. As industrialists in these cities became
the self-made economic and political elites of the Industrial Era,
Received Pronunciation lost its status and fizzled out in the U.S. The
prevalent accent in the Rust Belt, though, got dubbed General
American and spread across the states just as RP had in Britain.
Of course, with the speed that language changes, a General American
accent is now hard to find in much of this region, with New York,
Philadelphia, Pittsburgh, and Chicago developing their own unique
accents, and GenAm now considered generally confined to a small section
of the Midwest.
As mentioned above, there are regional exceptions to both these
general American and British sounds. Some of the accents of southeastern
England, plus the accents of Scotland and Ireland, are rhotic. Some
areas of the American Southeast, plus Boston, are non-rhotic.