diff --git a/lib/adapters/xhr.js b/lib/adapters/xhr.js
index 337b7a0..5575925 100644
--- a/lib/adapters/xhr.js
+++ b/lib/adapters/xhr.js
@@ -28,8 +28,10 @@ module.exports = function xhrAdapter(config) {
       }
     }
 
-    if (utils.isFormData(requestData) && utils.isStandardBrowserEnv()) {
-      delete requestHeaders['Content-Type']; // Let the browser set it
+
+    if (utils.isFormData(requestData) && utils.isStandardBrowserEnv() && !config.defineContentType ) {
+      // Let the browser set the content type should it not be set
+      delete requestHeaders['Content-Type'];
     }
 
     var request = new XMLHttpRequest();