diff --git a/requests/sessions.py b/requests/sessions.py
index ae4bcc8e79..3f59cab922 100644
--- a/requests/sessions.py
+++ b/requests/sessions.py
@@ -29,7 +29,7 @@
 
 from .utils import (
     requote_uri, get_environ_proxies, get_netrc_auth, should_bypass_proxies,
-    get_auth_from_url, rewind_body
+    get_auth_from_url, rewind_body, resolve_proxies
 )
 
 from .status_codes import codes
@@ -269,7 +269,6 @@ def rebuild_auth(self, prepared_request, response):
         if new_auth is not None:
             prepared_request.prepare_auth(new_auth)
 
-
     def rebuild_proxies(self, prepared_request, proxies):
         """This method re-evaluates the proxy configuration by considering the
         environment variables. If we are redirected to a URL covered by
@@ -282,21 +281,9 @@ def rebuild_proxies(self, prepared_request, proxies):
 
         :rtype: dict
         """
-        proxies = proxies if proxies is not None else {}
         headers = prepared_request.headers
-        url = prepared_request.url
-        scheme = urlparse(url).scheme
-        new_proxies = proxies.copy()
-        no_proxy = proxies.get('no_proxy')
-
-        bypass_proxy = should_bypass_proxies(url, no_proxy=no_proxy)
-        if self.trust_env and not bypass_proxy:
-            environ_proxies = get_environ_proxies(url, no_proxy=no_proxy)
-
-            proxy = environ_proxies.get(scheme, environ_proxies.get('all'))
-
-            if proxy:
-                new_proxies.setdefault(scheme, proxy)
+        scheme = urlparse(prepared_request.url).scheme
+        new_proxies = resolve_proxies(prepared_request, proxies, self.trust_env)
 
         if 'Proxy-Authorization' in headers:
             del headers['Proxy-Authorization']
@@ -633,7 +620,10 @@ def send(self, request, **kwargs):
         kwargs.setdefault('stream', self.stream)
         kwargs.setdefault('verify', self.verify)
         kwargs.setdefault('cert', self.cert)
-        kwargs.setdefault('proxies', self.rebuild_proxies(request, self.proxies))
+        if 'proxies' not in kwargs:
+            kwargs['proxies'] = resolve_proxies(
+                request, self.proxies, self.trust_env
+            )
 
         # It's possible that users might accidentally send a Request object.
         # Guard against that specific failure case.
diff --git a/requests/utils.py b/requests/utils.py
index 41bfb82fe0..53c282dadc 100644
--- a/requests/utils.py
+++ b/requests/utils.py
@@ -830,6 +830,34 @@ def select_proxy(url, proxies):
     return proxy
 
 
+def resolve_proxies(request, proxies, trust_env=True):
+    """This method takes proxy information from a request and configuration
+    input to resolve a mapping of target proxies. This will consider settings
+    such a NO_PROXY to strip proxy configurations.
+
+    :param request: Request or PreparedRequest
+    :param proxies: A dictionary of schemes or schemes and hosts to proxy URLs
+    :param trust_env: Boolean declaring whether to trust environment configs
+
+    :rtype: dict
+    """
+    proxies = proxies if proxies is not None else {}
+    url = request.url
+    scheme = urlparse(url).scheme
+    no_proxy = proxies.get('no_proxy')
+    new_proxies = proxies.copy()
+
+    bypass_proxy = should_bypass_proxies(url, no_proxy=no_proxy)
+    if trust_env and not bypass_proxy:
+        environ_proxies = get_environ_proxies(url, no_proxy=no_proxy)
+
+        proxy = environ_proxies.get(scheme, environ_proxies.get('all'))
+
+        if proxy:
+            new_proxies.setdefault(scheme, proxy)
+    return new_proxies
+
+
 def default_user_agent(name="python-requests"):
     """
     Return a string representing the default user agent.
diff --git a/tests/test_requests.py b/tests/test_requests.py
index b6d97dd9f4..463e8bf47a 100644
--- a/tests/test_requests.py
+++ b/tests/test_requests.py
@@ -590,6 +590,15 @@ def test_respect_proxy_env_on_request(self, httpbin):
                 session = requests.Session()
                 session.request(method='GET', url=httpbin())
 
+    def test_proxy_authorization_preserved_on_request(self, httpbin):
+        proxy_auth_value = "Bearer XXX"
+        session = requests.Session()
+        session.headers.update({"Proxy-Authorization": proxy_auth_value})
+        resp = session.request(method='GET', url=httpbin('get'))
+        sent_headers = resp.json().get('headers', {})
+
+        assert sent_headers.get("Proxy-Authorization") == proxy_auth_value
+
     def test_basicauth_with_netrc(self, httpbin):
         auth = ('user', 'pass')
         wrong_auth = ('wronguser', 'wrongpass')
@@ -2575,4 +2584,4 @@ def test_post_json_nan(self, httpbin):
     def test_json_decode_compatibility(self, httpbin):
         r = requests.get(httpbin('bytes/20'))
         with pytest.raises(requests.exceptions.JSONDecodeError):
-            r.json()
\ No newline at end of file
+            r.json()