Schema
You can download the schema file for an API version by using this URI:
Web_Services_SOAP_endpoint_hostname/services/async/APIversion/AsyncApi.xsd
For example, if you're working with version 63.0 of the Bulk API, the URI for a production org is in this format.
https://MyDomainName.my.salesforce.com/services/async/63.0/AsyncApi.xsd
You can find the My Domain name and My Domain login URL for your org on the My Domain page in Setup. Or, to get the hostname of your My Domain login URL in Apex, use the getMyDomainHostname() method of the System.DomainCreator class.
Schema and API Versions
The schema file is available for API versions earlier than the current release. You can download the schema file for API version 18.0 and later. For example, if you want to download the schema file for API version 31.0, use this URI:
https://MyDomainName.my.salesforce.com/services/async/31.0/AsyncApi.xsd