Ant's -D Option Is Not Always a JVM Proxy Option
An Ant property and a Java system property can have the same spelling without being the same setting. Configure the JVM or task that makes the request.
You added -Dhttp.proxyHost=... to an Ant command, but the download still goes directly. The spelling looks like a Java system property; the command receiving it matters.
Ant's command-line -D syntax defines Ant build properties. That is not automatically the same as passing a startup option to the JVM running Ant.
Ant documents ANT_OPTS for options supplied to that JVM. It also provides the setproxy task for supported proxy configuration from a build. Choose the interface matching the requirement and the client performing the request.
A proxy applied to Ant's JVM does not automatically configure a separate executable or independently configured client launched by a task. If a forked program fails, inspect its own command and environment.
For an allowed public download, the HTTP proxy list supplies candidates for the chosen interface's host and port. Keep machine-specific endpoints and account secrets out of shared build files where possible. A build that works for one developer should not force every other user to contact that developer's office proxy.
When reporting the failure, show where the setting was supplied: Ant property, JVM startup option, task configuration, or child-process environment. That distinction is often the missing explanation.