The legislation which created AONBs was the National Parks and Access to the Countryside Act of 1949. This act came about after the Second World War in response to increasing pressure for new development. The government of the day decided to formally recognise the fact that the countryside of England and Wales has a rich diversity of scenery, which is of great value and worthy of protection.
This led to the first AONB designation in 1956 - the Gower Peninsula. Two years later in 1958, the Northumberland Coast was designated as an AONB.
Ray Woolmore has written an interesting document chronicling the processes and decisions that led up to Northumberland Coast being finally designated in 1958. It provides a useful that explains why the AONB boundary is what it is today.
Read the designation history