diff --git a/.circleci/run.sh b/.circleci/run.sh
index 4798b8b386..bb4d4ac52b 100755
--- a/.circleci/run.sh
+++ b/.circleci/run.sh
@@ -157,7 +157,7 @@ check_clean_git()
     rm -f compiler/test/compilable/vcg-ast.d.cg
     rm -f compiler/test/compilable/vcg-ast-arraylength.d.cg
     # Ensure that there are no untracked changes
-    make -f posix.mak check-clean-git
+    make check-clean-git
 }
 
 # sanitycheck for the run_individual_tests script
diff --git a/posix.mak b/Makefile
similarity index 64%
rename from posix.mak
rename to Makefile
index 419066069b..3c530452c8 100644
--- a/posix.mak
+++ b/Makefile
@@ -1,20 +1,33 @@
-INSTALL_DIR=$(PWD)/../install
+include compiler/src/osmodel.mak
+
+INSTALL_DIR=$(shell pwd)/../install
 ECTAGS_LANGS = Make,C,C++,D,Sh
 ECTAGS_FILES = compiler/dmd/*.[chd] compiler/dmd/backend/*.[chd] compiler/dmd/root/*.[chd]
 
-GENERATED = generated
-HOST_DMD?=dmd
+EXE=$(if $(findstring windows,$(OS)),.exe,)
+
+HOST_DMD?=$(DMD)
+ifeq (,$(HOST_DMD))
+    HOST_DMD=dmd$(EXE)
+endif
+export HOST_DMD
+
+GENERATED=generated
+BUILD_EXE=$(GENERATED)/build$(EXE)
+RUN_EXE=$(GENERATED)/run$(EXE)
 
 .PHONY: all clean test install auto-tester-build auto-tester-test toolchain-info
 
-all: $(GENERATED)/build
-	$(GENERATED)/build dmd
+all: $(BUILD_EXE)
+	$(BUILD_EXE) dmd
+ifneq (windows,$(OS))
 	$(QUIET)$(MAKE) -C druntime -f posix.mak target
+endif
 
-$(GENERATED)/build: compiler/src/build.d
+$(BUILD_EXE): compiler/src/build.d
 	$(HOST_DMD) -of$@ -g $<
 
-$(GENERATED)/run: compiler/test/run.d
+$(RUN_EXE): compiler/test/run.d
 	$(HOST_DMD) -of$@ -g -i -Icompiler/test -release $<
 
 auto-tester-build:
@@ -25,35 +38,39 @@ auto-tester-test:
 
 buildkite-test: test
 
-toolchain-info: $(GENERATED)/build
-	$(GENERATED)/build $@
+toolchain-info: $(BUILD_EXE)
+	$(BUILD_EXE) $@
 
 clean:
-	rm -Rf $(GENERATED)
+	rm -rf $(GENERATED)
 	cd compiler/test && rm -rf test_results *.lst trace.log trace.def
 	$(RM) tags
+ifneq (windows,$(OS))
 	$(QUIET)$(MAKE) -C druntime -f posix.mak clean
+endif
 
-test: all $(GENERATED)/build $(GENERATED)/run
-	$(GENERATED)/build unittest
-	$(GENERATED)/run --environment HOST_DMD=$(HOST_DMD)
+test: all $(BUILD_EXE) $(RUN_EXE)
+	$(BUILD_EXE) unittest
+	$(RUN_EXE) --environment
+ifneq (windows,$(OS))
 	$(QUIET)$(MAKE) -C druntime -f posix.mak unittest
+endif
 
-html: $(GENERATED)/build
-	$(GENERATED)/build $@
+html: $(BUILD_EXE)
+	$(BUILD_EXE) $@
 
 # Creates Exuberant Ctags tags file
-tags: posix.mak $(ECTAGS_FILES)
+tags: Makefile $(ECTAGS_FILES)
 	ctags --sort=yes --links=no --excmd=number --languages=$(ECTAGS_LANGS) \
-		--langmap='C++:+.c,C++:+.h' --extra=+f --file-scope=yes --fields=afikmsSt --totals=yes posix.mak $(ECTAGS_FILES)
+		--langmap='C++:+.c,C++:+.h' --extra=+f --file-scope=yes --fields=afikmsSt --totals=yes Makefile $(ECTAGS_FILES)
 
 ifneq (,$(findstring Darwin_64_32, $(PWD)))
 install:
 	echo "Darwin_64_32_disabled"
 else
-install: all $(GENERATED)/build
-	$(GENERATED)/build man
-	$(GENERATED)/build install INSTALL_DIR=$(INSTALL_DIR)
+install: all $(BUILD_EXE)
+	$(BUILD_EXE) man
+	$(BUILD_EXE) install INSTALL_DIR=$(if $(findstring $(OS),windows),$(shell cygpath -w "$(INSTALL_DIR)"),$(INSTALL_DIR))
 	cp -r compiler/samples $(INSTALL_DIR)
 	mkdir -p $(INSTALL_DIR)/man
 	cp -r generated/docs/man/* $(INSTALL_DIR)/man/
@@ -69,8 +86,8 @@ check-clean-git:
 		exit 1; \
 	fi
 
-style: $(GENERATED)/build
-	$(GENERATED)/build $@
+style: $(BUILD_EXE)
+	$(BUILD_EXE) $@
 
 .DELETE_ON_ERROR: # GNU Make directive (delete output files on error)
 
diff --git a/compiler/src/osmodel.mak b/compiler/src/osmodel.mak
index 353ec80f2f..3c507e43ac 100644
--- a/compiler/src/osmodel.mak
+++ b/compiler/src/osmodel.mak
@@ -2,44 +2,47 @@
 #
 # Detects and sets the macros:
 #
-#   OS         = one of {osx,linux,freebsd,openbsd,netbsd,dragonflybsd,solaris}
+#   OS         = one of {windows,osx,linux,freebsd,openbsd,netbsd,dragonflybsd,solaris}
 #   MODEL      = one of { 32, 64 }
 #   MODEL_FLAG = one of { -m32, -m64 }
 #   ARCH       = one of { x86, x86_64, aarch64 }
 #
-# Note:
-#   Keep this file in sync between druntime, phobos, and dmd repositories!
-# Source: https://github.com/dlang/dmd/blob/master/src/osmodel.mak
+# On Windows, also sets up a bash shell.
 
 
 ifeq (,$(OS))
-  uname_S:=$(shell uname -s)
-  ifeq (Darwin,$(uname_S))
-    OS:=osx
-  endif
-  ifeq (Linux,$(uname_S))
-    OS:=linux
-  endif
-  ifeq (FreeBSD,$(uname_S))
-    OS:=freebsd
-  endif
-  ifeq (OpenBSD,$(uname_S))
-    OS:=openbsd
-  endif
-  ifeq (NetBSD,$(uname_S))
-    OS:=netbsd
-  endif
-  ifeq (DragonFly,$(uname_S))
-    OS:=dragonflybsd
-  endif
-  ifeq (Solaris,$(uname_S))
-    OS:=solaris
-  endif
-  ifeq (SunOS,$(uname_S))
-    OS:=solaris
-  endif
-  ifeq (,$(OS))
-    $(error Unrecognized or unsupported OS for uname: $(uname_S))
+  ifneq (,$(LOCALAPPDATA))
+    # assume Windows
+    OS:=windows
+  else
+    uname_S:=$(shell uname -s)
+    ifeq (Darwin,$(uname_S))
+      OS:=osx
+    endif
+    ifeq (Linux,$(uname_S))
+      OS:=linux
+    endif
+    ifeq (FreeBSD,$(uname_S))
+      OS:=freebsd
+    endif
+    ifeq (OpenBSD,$(uname_S))
+      OS:=openbsd
+    endif
+    ifeq (NetBSD,$(uname_S))
+      OS:=netbsd
+    endif
+    ifeq (DragonFly,$(uname_S))
+      OS:=dragonflybsd
+    endif
+    ifeq (Solaris,$(uname_S))
+      OS:=solaris
+    endif
+    ifeq (SunOS,$(uname_S))
+      OS:=solaris
+    endif
+    ifeq (,$(OS))
+      $(error Unrecognized or unsupported OS for uname: $(uname_S))
+    endif
   endif
 endif
 
@@ -49,26 +52,45 @@ ifeq (MACOS,$(OS))
   OS:=osx
 endif
 
+# Windows predefines OS to e.g. `Windows_NT`
+ifneq (,$(findstring Win,$(OS)))
+  OS:=windows
+endif
+
+# set up bash shell on Windows
+ifeq (windows,$(OS))
+  # Note: setting SHELL to an absolute path to bash.exe does NOT suffice.
+  # The GNU tools like {rm,cp,mkdir}.exe need to be in PATH.
+  export PATH:=C:\Program Files\Git\usr\bin;$(PATH)
+  SHELL=bash.exe
+  $(info Using make SHELL "$(SHELL)", should be bash.)
+endif
+
 ifeq (,$(MODEL))
-  ifeq ($(OS), solaris)
-    uname_M:=$(shell isainfo -n)
-  else
-    uname_M:=$(shell uname -m)
-  endif
-  ifneq (,$(findstring $(uname_M),x86_64 amd64))
+  ifeq (windows,$(OS))
     MODEL:=64
     ARCH:=x86_64
-  endif
-  ifneq (,$(findstring $(uname_M),aarch64 arm64))
-    MODEL:=64
-    ARCH:=aarch64
-  endif
-  ifneq (,$(findstring $(uname_M),i386 i586 i686))
-    MODEL:=32
-    ARCH:=x86
-  endif
-  ifeq (,$(MODEL))
-    $(error Cannot figure 32/64 model and arch from uname -m: $(uname_M))
+  else
+    ifeq ($(OS), solaris)
+      uname_M:=$(shell isainfo -n)
+    else
+      uname_M:=$(shell uname -m)
+    endif
+    ifneq (,$(findstring $(uname_M),x86_64 amd64))
+      MODEL:=64
+      ARCH:=x86_64
+    endif
+    ifneq (,$(findstring $(uname_M),aarch64 arm64))
+      MODEL:=64
+      ARCH:=aarch64
+    endif
+    ifneq (,$(findstring $(uname_M),i386 i586 i686))
+      MODEL:=32
+      ARCH:=x86
+    endif
+    ifeq (,$(MODEL))
+      $(error Cannot figure 32/64 model and arch from uname -m: $(uname_M))
+    endif
   endif
 endif
 
diff --git a/druntime/posix.mak b/druntime/posix.mak
index 3a5d1b9295..28023dadc8 100644
--- a/druntime/posix.mak
+++ b/druntime/posix.mak
@@ -351,7 +351,7 @@ $(IMPDIR)/%.h : src/%.h
 ######################## Build DMD if non-existent ##############################
 
 $(DMD):
-	$(MAKE) -C .. -f posix.mak generated/build
+	$(MAKE) -C .. generated/build DMD=""
 	../generated/build dmd BUILD=$(BUILD) OS=$(OS) MODEL=$(MODEL)
 
 ################### C/ASM Targets ############################
diff --git a/src/posix.mak b/src/posix.mak
index ce52f955e3..eab8d0227d 100644
--- a/src/posix.mak
+++ b/src/posix.mak
@@ -2,31 +2,31 @@
 
 
 all:
-	$(QUIET)$(MAKE) -C .. -f posix.mak $@
+	$(QUIET)$(MAKE) -C .. $@
 
 buildkite-test:
-	$(QUIET)$(MAKE) -C .. -f posix.mak $@
+	$(QUIET)$(MAKE) -C .. $@
 
 toolchain-info:
-	$(QUIET)$(MAKE) -C .. -f posix.mak $@
+	$(QUIET)$(MAKE) -C .. $@
 
 clean:
-	$(QUIET)$(MAKE) -C .. -f posix.mak $@
+	$(QUIET)$(MAKE) -C .. $@
 
 test:
-	$(QUIET)$(MAKE) -C .. -f posix.mak $@
+	$(QUIET)$(MAKE) -C .. $@
 
 html:
-	$(QUIET)$(MAKE) -C .. -f posix.mak $@
+	$(QUIET)$(MAKE) -C .. $@
 
 tags:
-	$(QUIET)$(MAKE) -C .. -f posix.mak $@
+	$(QUIET)$(MAKE) -C .. $@
 
 install:
-	$(QUIET)$(MAKE) -C .. -f posix.mak $@
+	$(QUIET)$(MAKE) -C .. $@
 
 check-clean-git:
-	$(QUIET)$(MAKE) -C .. -f posix.mak $@
+	$(QUIET)$(MAKE) -C .. $@
 
 style:
-	$(QUIET)$(MAKE) -C .. -f posix.mak $@
+	$(QUIET)$(MAKE) -C .. $@