diff --git a/src/unit.cpp b/src/unit.cpp
index c140c5ec512..c4d4ffd356f 100644
--- a/src/unit.cpp
+++ b/src/unit.cpp
@@ -838,19 +838,15 @@ void unit::advance_to(const config &old_cfg, const unit_type &u_type,
 
 	if ( new_type.movement_type().get_parent() ) {
 		new_cfg.merge_with(new_type.movement_type().get_parent()->get_cfg());
+		// Convert movement type's "flies" to unit's "flying".
+		if ( const config::attribute_value * flies = new_cfg.get("flies") ) {
+			new_cfg["flying"] = flies->to_bool();
+			new_cfg.remove_attribute("flies");
+		}
 	}
 
-	new_cfg.merge_with(new_type.get_cfg());
-
-	// Remove "pure" unit_type attributes (attributes that do not get directly
-	// copied to units; some do get copied, but under different keys).
-	static char const *unit_type_attrs[] = { "attacks", "die_sound",
-		"experience", "flies", "hide_help", "hitpoints", "id",
-		"ignore_race_traits", "inherit", "movement", "movement_type",
-		"name", "num_traits", "variation_name" };
-	BOOST_FOREACH(const char *attr, unit_type_attrs) {
-		new_cfg.remove_attribute(attr);
-	}
+	// Inherit from the new unit type.
+	new_cfg.merge_with(new_type.get_cfg_for_units());
 
 	// If unit has specific profile, remember it and keep it after advancing
 	std::string profile = old_cfg["profile"].str();
@@ -867,8 +863,6 @@ void unit::advance_to(const config &old_cfg, const unit_type &u_type,
 	}
 
 	cfg_.swap(new_cfg);
-	cfg_.clear_children("male");
-	cfg_.clear_children("female");
 	// NOTE: There should be no need to access old_cfg (or new_cfg) after this
 	//       line. Particularly since the swap might have affected old_cfg.
 
diff --git a/src/unit_types.cpp b/src/unit_types.cpp
index 29a79a2248a..b78f8ed88bf 100644
--- a/src/unit_types.cpp
+++ b/src/unit_types.cpp
@@ -620,6 +620,8 @@ int defense_modifier_internal(defense_cache &defense_mods,
 
 unit_type::unit_type(const unit_type& o) :
 	cfg_(o.cfg_),
+	unit_cfg_(),  // Not copied; will be re-created if needed.
+	built_unit_cfg_(false),
 	id_(o.id_),
 	debug_id_(o.debug_id_),
 	base_id_(o.base_id_),
@@ -671,6 +673,8 @@ unit_type::unit_type(const unit_type& o) :
 
 unit_type::unit_type(const config &cfg, const std::string & parent_id) :
 	cfg_(cfg),
+	unit_cfg_(),
+	built_unit_cfg_(false),
 	id_(cfg_.has_attribute("id") ? cfg_["id"].str() : parent_id),
 	debug_id_(),
 	base_id_(!parent_id.empty() ? parent_id : id_),
@@ -1310,6 +1314,33 @@ std::vector<std::string> unit_type::variations() const
 }
 
 
+/**
+ * Generates (and returns) a trimmed config suitable for use with units.
+ */
+const config & unit_type::build_unit_cfg() const
+{
+	// We start with everything.
+	unit_cfg_ = cfg_;
+
+	// Remove "pure" unit_type attributes (attributes that do not get directly
+	// copied to units; some do get copied, but under different keys).
+	static char const *unit_type_attrs[] = { "attacks", "die_sound",
+		"experience", "flies", "hide_help", "hitpoints", "id",
+		"ignore_race_traits", "inherit", "movement", "movement_type",
+		"name", "num_traits", "variation_name" };
+	BOOST_FOREACH(const char *attr, unit_type_attrs) {
+		unit_cfg_.remove_attribute(attr);
+	}
+
+	// Remove gendered children.
+	unit_cfg_.clear_children("male");
+	unit_cfg_.clear_children("female");
+
+	built_unit_cfg_ = true;
+	return unit_cfg_;
+}
+
+
 /* ** unit_type_data ** */
 
 
diff --git a/src/unit_types.hpp b/src/unit_types.hpp
index 288960ee4e0..edd3f07f1ca 100644
--- a/src/unit_types.hpp
+++ b/src/unit_types.hpp
@@ -327,11 +327,19 @@ public:
 	const std::vector<tportrait>& portraits() const { return portraits_; }
 
 	const config &get_cfg() const { return cfg_; }
+	/// Returns a trimmed config suitable for use with units.
+	const config & get_cfg_for_units() const
+	{ return built_unit_cfg_ ? unit_cfg_ : build_unit_cfg(); }
+private:
+	/// Generates (and returns) a trimmed config suitable for use with units.
+	const config & build_unit_cfg() const;
 
 private:
 	void operator=(const unit_type& o);
 
 	const config &cfg_;
+	mutable config unit_cfg_;  /// Generated as needed via get_cfg_for_units().
+	mutable bool built_unit_cfg_;
 
 	std::string id_;
 	std::string debug_id_;  /// A suffix for id_, used when logging messages.