diff --git a/WHATSNEW b/WHATSNEW
index 38e258017..dcf8e9855 100644
--- a/WHATSNEW
+++ b/WHATSNEW
@@ -40,6 +40,8 @@ Fixed bugs:
 
 * Synchonization issues in PropertyHelper.  Bugzilla 41353.
 
+* <concat binary="true" append="true"> did not append.  Bugzilla 41399.
+
 Other changes:
 --------------
 
diff --git a/src/main/org/apache/tools/ant/taskdefs/Concat.java b/src/main/org/apache/tools/ant/taskdefs/Concat.java
index a9580a849..f996721ea 100644
--- a/src/main/org/apache/tools/ant/taskdefs/Concat.java
+++ b/src/main/org/apache/tools/ant/taskdefs/Concat.java
@@ -479,7 +479,7 @@ public class Concat extends Task {
         InputStream in = null;
         try {
             try {
-                out = new FileOutputStream(destinationFile);
+                out = new FileOutputStream(destinationFile, append);
             } catch (Exception t) {
                 throw new BuildException("Unable to open "
                     + destinationFile + " for writing", t);
diff --git a/src/tests/antunit/taskdefs/concat-test.xml b/src/tests/antunit/taskdefs/concat-test.xml
new file mode 100644
index 000000000..3bc2536ea
--- /dev/null
+++ b/src/tests/antunit/taskdefs/concat-test.xml
@@ -0,0 +1,24 @@
+<project xmlns:au="antlib:org.apache.ant.antunit">
+  <target name="tearDown">
+    <delete file="binaryAppendDest" />
+  </target>
+
+  <target name="testBinaryAppend">
+    <au:assertTrue message="prerequisite conditions unmet">
+      <length length="0">
+        <!-- allow for nonexistent OR zero-length: -->
+        <fileset file="binaryAppendDest" />
+      </length>
+    </au:assertTrue>
+    <echo file="binaryAppendDest">x</echo>
+    <au:assertTrue message="destfile setup failed">
+      <length length="1" file="binaryAppendDest" />
+    </au:assertTrue>
+    <concat append="true" destfile="binaryAppendDest" binary="true">
+      <string value="x" />
+    </concat>
+    <au:assertTrue message="expected length 2">
+      <length file="binaryAppendDest" length="2" />
+    </au:assertTrue>
+  </target>
+</project>