Original: The Kingdom of Great Britain came into being on 1 May 1707, as a result of the political union of the Kingdom of England (which included Wales) and the Kingdom of Scotland.
Simplified: It began to take its present shape with the Acts of Union in 1707, which united the crowns and Parliaments of England and Scotland to create the Kingdom of Great Britain. A further Act of Union in 1800 joined the Kingdom of Great Britain and the Kingdom of Ireland to create the United Kingdom of Great Britain and Ireland.