1. Internal Jeff Bezos memo from around 2002 that everything should be a web service, which eventually can be externalize: http://apievangelist.com/2012/01/12/the-secret-to-amazons-su...
"Anyone who doesn’t do this will be fired. Thank you; have a nice day!"
2. AWS started in 2006 with EC2 (provision service through API) and S3 (some file storage). Simple, but primitive building blocks.
Google entered game early in 2008 with App Engine. Very powerful, but restricted Platform as a Service. It pretty much requires to rewrite whole application from scratch and doesn't let you do tons of stuff.
Eventually AWS is keep adding services with more functionality (SimpleDB, DynamoDB, SQS), while it took quite a while Google to realize that it needs to provide bare servers too. So both approaches converge, but Amazon capture magnitude more revenues from cloud infrastructure.