diff --git a/awc/CHANGES.md b/awc/CHANGES.md
index a4948c827..8a3fea46a 100644
--- a/awc/CHANGES.md
+++ b/awc/CHANGES.md
@@ -1,6 +1,9 @@
 # Changes
 
 ## Unreleased - 2021-xx-xx
+* Add `ClientBuilder::add_default_header` and deprecate `ClientBuilder::header`. [#2510]
+
+[#2510]: https://github.com/actix/actix-web/pull/2510
 
 
 ## 3.0.0-beta.13 - 2021-12-11
diff --git a/awc/src/builder.rs b/awc/src/builder.rs
index e9c1319fd..09f8063bb 100644
--- a/awc/src/builder.rs
+++ b/awc/src/builder.rs
@@ -2,7 +2,7 @@ use std::{convert::TryFrom, fmt, net::IpAddr, rc::Rc, time::Duration};
 
 use actix_http::{
     error::HttpError,
-    header::{self, HeaderMap, HeaderName},
+    header::{self, HeaderMap, HeaderName, TryIntoHeaderPair},
     Uri,
 };
 use actix_rt::net::{ActixStream, TcpStream};
@@ -21,11 +21,11 @@ use crate::{
 /// This type can be used to construct an instance of `Client` through a
 /// builder-like pattern.
 pub struct ClientBuilder<S = (), M = ()> {
-    default_headers: bool,
     max_http_version: Option<http::Version>,
     stream_window_size: Option<u32>,
     conn_window_size: Option<u32>,
-    headers: HeaderMap,
+    fundamental_headers: bool,
+    default_headers: HeaderMap,
     timeout: Option<Duration>,
     connector: Connector<S>,
     middleware: M,
@@ -44,15 +44,15 @@ impl ClientBuilder {
         (),
     > {
         ClientBuilder {
-            middleware: (),
-            default_headers: true,
-            headers: HeaderMap::new(),
-            timeout: Some(Duration::from_secs(5)),
-            local_address: None,
-            connector: Connector::new(),
             max_http_version: None,
             stream_window_size: None,
             conn_window_size: None,
+            fundamental_headers: true,
+            default_headers: HeaderMap::new(),
+            timeout: Some(Duration::from_secs(5)),
+            connector: Connector::new(),
+            middleware: (),
+            local_address: None,
             max_redirects: 10,
         }
     }
@@ -78,8 +78,8 @@ where
     {
         ClientBuilder {
             middleware: self.middleware,
+            fundamental_headers: self.fundamental_headers,
             default_headers: self.default_headers,
-            headers: self.headers,
             timeout: self.timeout,
             local_address: self.local_address,
             connector,
@@ -153,15 +153,31 @@ where
         self
     }
 
-    /// Do not add default request headers.
+    /// Do not add fundamental default request headers.
+    ///
     /// By default `Date` and `User-Agent` headers are set.
     pub fn no_default_headers(mut self) -> Self {
-        self.default_headers = false;
+        self.fundamental_headers = false;
         self
     }
 
-    /// Add default header. Headers added by this method
-    /// get added to every request.
+    /// Add default header.
+    ///
+    /// Headers added by this method get added to every request unless overriden by .
+    ///
+    /// # Panics
+    /// Panics if header name or value is invalid.
+    pub fn add_default_header(mut self, header: impl TryIntoHeaderPair) -> Self {
+        match header.try_into_header_pair() {
+            Ok((key, value)) => self.default_headers.append(key, value),
+            Err(err) => panic!("Header error: {:?}", err.into()),
+        }
+
+        self
+    }
+
+    #[doc(hidden)]
+    #[deprecated(since = "3.0.0", note = "Prefer `add_default_header((key, value))`.")]
     pub fn header<K, V>(mut self, key: K, value: V) -> Self
     where
         HeaderName: TryFrom<K>,
@@ -172,11 +188,11 @@ where
         match HeaderName::try_from(key) {
             Ok(key) => match value.try_into_value() {
                 Ok(value) => {
-                    self.headers.append(key, value);
+                    self.default_headers.append(key, value);
                 }
-                Err(e) => log::error!("Header value error: {:?}", e),
+                Err(err) => log::error!("Header value error: {:?}", err),
             },
-            Err(e) => log::error!("Header name error: {:?}", e),
+            Err(err) => log::error!("Header name error: {:?}", err),
         }
         self
     }
@@ -190,10 +206,10 @@ where
             Some(password) => format!("{}:{}", username, password),
             None => format!("{}:", username),
         };
-        self.header(
+        self.add_default_header((
             header::AUTHORIZATION,
             format!("Basic {}", base64::encode(&auth)),
-        )
+        ))
     }
 
     /// Set client wide HTTP bearer authentication header
@@ -201,13 +217,12 @@ where
     where
         T: fmt::Display,
     {
-        self.header(header::AUTHORIZATION, format!("Bearer {}", token))
+        self.add_default_header((header::AUTHORIZATION, format!("Bearer {}", token)))
     }
 
-    /// Registers middleware, in the form of a middleware component (type),
-    /// that runs during inbound and/or outbound processing in the request
-    /// life-cycle (request -> response), modifying request/response as
-    /// necessary, across all requests managed by the Client.
+    /// Registers middleware, in the form of a middleware component (type), that runs during inbound
+    /// and/or outbound processing in the request life-cycle (request -> response),
+    /// modifying request/response as necessary, across all requests managed by the `Client`.
     pub fn wrap<S1, M1>(
         self,
         mw: M1,
@@ -218,11 +233,11 @@ where
     {
         ClientBuilder {
             middleware: NestTransform::new(self.middleware, mw),
-            default_headers: self.default_headers,
+            fundamental_headers: self.fundamental_headers,
             max_http_version: self.max_http_version,
             stream_window_size: self.stream_window_size,
             conn_window_size: self.conn_window_size,
-            headers: self.headers,
+            default_headers: self.default_headers,
             timeout: self.timeout,
             connector: self.connector,
             local_address: self.local_address,
@@ -237,10 +252,10 @@ where
         M::Transform:
             Service<ConnectRequest, Response = ConnectResponse, Error = SendRequestError>,
     {
-        let redirect_time = self.max_redirects;
+        let max_redirects = self.max_redirects;
 
-        if redirect_time > 0 {
-            self.wrap(Redirect::new().max_redirect_times(redirect_time))
+        if max_redirects > 0 {
+            self.wrap(Redirect::new().max_redirect_times(max_redirects))
                 ._finish()
         } else {
             self._finish()
@@ -272,7 +287,7 @@ where
         let connector = boxed::rc_service(self.middleware.new_transform(connector));
 
         Client(ClientConfig {
-            headers: Rc::new(self.headers),
+            default_headers: Rc::new(self.default_headers),
             timeout: self.timeout,
             connector,
         })
@@ -288,7 +303,7 @@ mod tests {
         let client = ClientBuilder::new().basic_auth("username", Some("password"));
         assert_eq!(
             client
-                .headers
+                .default_headers
                 .get(header::AUTHORIZATION)
                 .unwrap()
                 .to_str()
@@ -299,7 +314,7 @@ mod tests {
         let client = ClientBuilder::new().basic_auth("username", None);
         assert_eq!(
             client
-                .headers
+                .default_headers
                 .get(header::AUTHORIZATION)
                 .unwrap()
                 .to_str()
@@ -313,7 +328,7 @@ mod tests {
         let client = ClientBuilder::new().bearer_auth("someS3cr3tAutht0k3n");
         assert_eq!(
             client
-                .headers
+                .default_headers
                 .get(header::AUTHORIZATION)
                 .unwrap()
                 .to_str()
diff --git a/awc/src/lib.rs b/awc/src/lib.rs
index 06fd33fac..00c559406 100644
--- a/awc/src/lib.rs
+++ b/awc/src/lib.rs
@@ -168,7 +168,7 @@ pub struct Client(ClientConfig);
 #[derive(Clone)]
 pub(crate) struct ClientConfig {
     pub(crate) connector: BoxConnectorService,
-    pub(crate) headers: Rc<HeaderMap>,
+    pub(crate) default_headers: Rc<HeaderMap>,
     pub(crate) timeout: Option<Duration>,
 }
 
@@ -204,7 +204,9 @@ impl Client {
     {
         let mut req = ClientRequest::new(method, url, self.0.clone());
 
-        for header in self.0.headers.iter() {
+        for header in self.0.default_headers.iter() {
+            // header map is empty
+            // TODO: probably append instead
             req = req.insert_header_if_none(header);
         }
         req
@@ -297,7 +299,7 @@ impl Client {
         <Uri as TryFrom<U>>::Error: Into<HttpError>,
     {
         let mut req = ws::WebsocketsRequest::new(url, self.0.clone());
-        for (key, value) in self.0.headers.iter() {
+        for (key, value) in self.0.default_headers.iter() {
             req.head.headers.insert(key.clone(), value.clone());
         }
         req
@@ -308,6 +310,6 @@ impl Client {
     /// Returns Some(&mut HeaderMap) when Client object is unique
     /// (No other clone of client exists at the same time).
     pub fn headers(&mut self) -> Option<&mut HeaderMap> {
-        Rc::get_mut(&mut self.0.headers)
+        Rc::get_mut(&mut self.0.default_headers)
     }
 }
diff --git a/awc/src/middleware/redirect.rs b/awc/src/middleware/redirect.rs
index 0fde48074..704d2d79d 100644
--- a/awc/src/middleware/redirect.rs
+++ b/awc/src/middleware/redirect.rs
@@ -442,13 +442,15 @@ mod tests {
         });
 
         let client = ClientBuilder::new()
-            .header("custom", "value")
+            .add_default_header(("custom", "value"))
             .disable_redirects()
             .finish();
         let res = client.get(srv.url("/")).send().await.unwrap();
         assert_eq!(res.status().as_u16(), 302);
 
-        let client = ClientBuilder::new().header("custom", "value").finish();
+        let client = ClientBuilder::new()
+            .add_default_header(("custom", "value"))
+            .finish();
         let res = client.get(srv.url("/")).send().await.unwrap();
         assert_eq!(res.status().as_u16(), 200);
 
@@ -520,7 +522,7 @@ mod tests {
 
         // send a request to different origins, http://srv1/ then http://srv2/. So it should remove the header
         let client = ClientBuilder::new()
-            .header(header::AUTHORIZATION, "auth_key_value")
+            .add_default_header((header::AUTHORIZATION, "auth_key_value"))
             .finish();
         let res = client.get(srv1.url("/")).send().await.unwrap();
         assert_eq!(res.status().as_u16(), 200);
diff --git a/awc/src/request.rs b/awc/src/request.rs
index bfb72f1db..21f187e99 100644
--- a/awc/src/request.rs
+++ b/awc/src/request.rs
@@ -579,7 +579,7 @@ mod tests {
     #[actix_rt::test]
     async fn test_client_header() {
         let req = Client::builder()
-            .header(header::CONTENT_TYPE, "111")
+            .add_default_header((header::CONTENT_TYPE, "111"))
             .finish()
             .get("/");
 
@@ -597,7 +597,7 @@ mod tests {
     #[actix_rt::test]
     async fn test_client_header_override() {
         let req = Client::builder()
-            .header(header::CONTENT_TYPE, "111")
+            .add_default_header((header::CONTENT_TYPE, "111"))
             .finish()
             .get("/")
             .insert_header((header::CONTENT_TYPE, "222"));
diff --git a/awc/src/ws.rs b/awc/src/ws.rs
index e32a9affb..06d54aadb 100644
--- a/awc/src/ws.rs
+++ b/awc/src/ws.rs
@@ -445,7 +445,7 @@ mod tests {
     #[actix_rt::test]
     async fn test_header_override() {
         let req = Client::builder()
-            .header(header::CONTENT_TYPE, "111")
+            .add_default_header((header::CONTENT_TYPE, "111"))
             .finish()
             .ws("/")
             .set_header(header::CONTENT_TYPE, "222");