XML-RPC was inspired by two earlier protocols. The first is an anonymous RPC protocol designed by Dave Winer and announced in an old DaveNet essay. (This is why XML-RPC servers are often installed under /RPC2.) The other, more important inspiration was an early draft of the SOAP protocol.
A longer history of XML-RPC has been generously provided by Dave Winer. This also explains the relationship between XML-RPC and SOAP.