diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index dd66a7cc52b..5f3807ec889 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -1,4 +1,4 @@
-name: Windows Rtools40
+name: Windows Rtools44
 
 on:
   pull_request:
@@ -30,16 +30,16 @@ jobs:
         python-version: '3.x'
     - uses: r-lib/actions/setup-r@v2
       with:
-        r-version: 4.1.3
+        r-version: 'release'
+        rtools-version: '44'
 
-    - name: Set path for Rtools40
+    - name: Set path for Rtools44
       if: runner.os == 'Windows'
-      run: echo "C:/rtools40/usr/bin;C:/rtools40/mingw64/bin" | Out-File -Append -FilePath $env:GITHUB_PATH -Encoding utf8
+      run: echo "C:/rtools44/usr/bin;C:/rtools44/x86_64-w64-mingw32.static.posix/bin" | Out-File -Append -FilePath $env:GITHUB_PATH -Encoding utf8
 
     - name: Build Math libs
       shell: powershell
       run: |
-        Add-Content make\local "O=1`n"
         make -f make/standalone math-libs -j2
     - name: Add TBB to PATH
       shell: powershell
@@ -71,16 +71,16 @@ jobs:
         python-version: '3.x'
     - uses: r-lib/actions/setup-r@v2
       with:
-        r-version: 4.1.3
+        r-version: 'release'
+        rtools-version: '44'
 
-    - name: Set path for Rtools40
+    - name: Set path for Rtools44
       if: runner.os == 'Windows'
-      run: echo "C:/rtools40/usr/bin;C:/rtools40/mingw64/bin" | Out-File -Append -FilePath $env:GITHUB_PATH -Encoding utf8
+      run: echo "C:/rtools44/usr/bin;C:/rtools44/x86_64-w64-mingw32.static.posix/bin" | Out-File -Append -FilePath $env:GITHUB_PATH -Encoding utf8
 
     - name: Build Math libs
       shell: powershell
       run: |
-        Add-Content make\local "O=1`n"
         make -f make/standalone math-libs -j2
     - name: Add TBB to PATH
       shell: powershell
@@ -116,16 +116,16 @@ jobs:
         python-version: '3.x'
     - uses: r-lib/actions/setup-r@v2
       with:
-        r-version: 4.1.3
+        r-version: 'release'
+        rtools-version: '44'
 
-    - name: Set path for Rtools40
+    - name: Set path for Rtools44
       if: runner.os == 'Windows'
-      run: echo "C:/rtools40/usr/bin;C:/rtools40/mingw64/bin" | Out-File -Append -FilePath $env:GITHUB_PATH -Encoding utf8
+      run: echo "C:/rtools44/usr/bin;C:/rtools44/x86_64-w64-mingw32.static.posix/bin" | Out-File -Append -FilePath $env:GITHUB_PATH -Encoding utf8
 
     - name: Build Math libs
       shell: powershell
       run: |
-        Add-Content make\local "O=1`n"
         make -f make/standalone math-libs -j2
     - name: Add TBB to PATH
       shell: powershell
@@ -156,11 +156,12 @@ jobs:
         python-version: '3.x'
     - uses: r-lib/actions/setup-r@v2
       with:
-        r-version: 4.1.3
+        r-version: 'release'
+        rtools-version: '44'
 
-    - name: Set path for Rtools40
+    - name: Set path for Rtools44
       if: runner.os == 'Windows'
-      run: echo "C:/rtools40/usr/bin;C:/rtools40/mingw64/bin" | Out-File -Append -FilePath $env:GITHUB_PATH -Encoding utf8
+      run: echo "C:/rtools44/usr/bin;C:/rtools44/x86_64-w64-mingw32.static.posix/bin" | Out-File -Append -FilePath $env:GITHUB_PATH -Encoding utf8
 
     - name: Build Math libs
       shell: powershell
diff --git a/make/compiler_flags b/make/compiler_flags
index 5a3a703f07f..e3f413c7579 100644
--- a/make/compiler_flags
+++ b/make/compiler_flags
@@ -161,6 +161,7 @@ ifeq ($(OS),Windows_NT)
   CXXFLAGS_WARNINGS ?= -Wall -Wno-unused-function -Wno-uninitialized -Wno-unused-but-set-variable -Wno-unused-variable -Wno-sign-compare -Wno-unused-local-typedefs -Wno-int-in-bool-context -Wno-attributes
   CPPFLAGS_GTEST ?= -DGTEST_HAS_PTHREAD=0
   CPPFLAGS_OS ?= -D_USE_MATH_DEFINES
+  CPPFLAGS_OS += -D_GLIBCXX11_USE_C99_COMPLEX
   ## Detecting Process Bitness:
   ## http://blogs.msdn.com/b/david.wang/archive/2006/03/26/howto-detect-process-bitness.aspx
   ifeq ($(PROCESSOR_ARCHITECTURE)$(PROCESSOR_ARCHITEW6432),x86)