I found an item on the FAQ section of Lightbend Activator useful. I am using Activator, which in turn uses SBT, so not sure if this helps users with just SBT, but if you use Activator, like me, and are behind a proxy, follow the instructions in the "Behind A Proxy" section of the FAQ:
https://www.lightbend.com/activator/docs
Just in case the content disappears, here's a copy-paste:
When running activator behind a proxy, some additional configuration is needed. First, open the activator configuration file, found in your user home directory under ~/.activator/activatorconfig.txt. Note that this file may not exist. Add the following lines (one option per line):
-Dhttp.proxyHost=PUT YOUR PROXY HOST HERE
-Dhttp.proxyPort=PUT YOUR PROXY PORT HERE
-Dhttp.nonProxyHosts="localhost|127.0.0.1"
-Dhttps.proxyHost=PUT YOUR HTTPS PROXY HOST HERE
-Dhttps.proxyPort=PUT YOUR HTTPS PROXY PORT HERE
-Dhttps.nonProxyHosts="localhost|127.0.0.1"