diff -u b/src/Entity/MediaBundle.php b/src/Entity/MediaBundle.php
--- b/src/Entity/MediaBundle.php
+++ b/src/Entity/MediaBundle.php
@@ -296,7 +296,6 @@
               ->save();
           }
         }
-
         $field->save();
       }
     }
diff -u b/src/MediaTypeBase.php b/src/MediaTypeBase.php
--- b/src/MediaTypeBase.php
+++ b/src/MediaTypeBase.php
@@ -7,7 +7,6 @@
 use Drupal\Core\Config\Config;
 use Drupal\Core\Entity\EntityFieldManagerInterface;
 use Drupal\Core\Entity\EntityTypeManagerInterface;
-use Drupal\Core\Field\FieldDefinitionInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
 use Drupal\Core\StringTranslation\StringTranslationTrait;
@@ -143,12 +142,9 @@
   public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
     $options = [];
 
-    /** @var \Drupal\media_entity\MediaBundleInterface $bundle */
-    $bundle = $form_state->getFormObject()->getEntity();
-    foreach ($this->entityFieldManager->getFieldDefinitions('media', $bundle->id()) as $field_name => $field) {
+    foreach ($this->entityFieldManager->getFieldStorageDefinitions('media') as $field_name => $field) {
       $allowed_type = in_array($field->getType(), $this->pluginDefinition['allowed_field_types']);
-      $is_base_field = $field->getFieldStorageDefinition()->isBaseField();
-      if ($allowed_type && !$is_base_field) {
+      if ($allowed_type && !$field->isBaseField()) {
         $options[$field_name] = $field->getLabel();
       }
     }
@@ -189,6 +185,9 @@
    * {@inheritdoc}
    */
   public function getSourceField(MediaBundleInterface $bundle) {
+    if (empty($this->configuration['source_field'])) {
+      return $this->createSourceField($bundle);
+    }
     $id = 'media.' . $bundle->id() . '.' . $this->configuration['source_field'];
 
     return $this->entityTypeManager
@@ -199,6 +198,33 @@
   }
 
   /**
+   * Returns the source field storage definition.
+   *
+   * @return \Drupal\field\FieldStorageConfigInterface
+   *   The field storage definition. Will be unsaved if new.
+   */
+  protected function getSourceFieldStorage() {
+    if ($this->configuration['source_field']) {
+      $id = 'media.' . $this->configuration['source_field'];
+
+      return $this->entityTypeManager
+        ->getStorage('field_storage_config')
+        ->load($id);
+    }
+    else {
+      return $this->createSourceFieldStorage();
+    }
+  }
+
+  /**
+   * Creates the source field storage definition.
+   *
+   * @return \Drupal\field\FieldStorageConfigInterface
+   *   The unsaved field storage definition.
+   */
+  abstract protected function createSourceFieldStorage();
+
+  /**
    * Creates the source field definition for a bundle.
    *
    * @param \Drupal\media_entity\MediaBundleInterface $bundle
@@ -206,7 +232,7 @@
    *
    * @return \Drupal\field\FieldConfigInterface
    *   The unsaved field definition. The field storage definition, if new,
-   *   should not be saved either.
+   *   should also be unsaved.
    */
   abstract protected function createSourceField(MediaBundleInterface $bundle);
 
diff -u b/src/Plugin/MediaEntity/Type/Generic.php b/src/Plugin/MediaEntity/Type/Generic.php
--- b/src/Plugin/MediaEntity/Type/Generic.php
+++ b/src/Plugin/MediaEntity/Type/Generic.php
@@ -13,7 +13,8 @@
  * @MediaType(
  *   id = "generic",
  *   label = @Translation("Generic media"),
- *   description = @Translation("Generic media type.")
+ *   description = @Translation("Generic media type."),
+ *   allowed_field_types = {"string"}
  * )
  */
 class Generic extends MediaTypeBase {
@@ -56,8 +57,8 @@
   /**
    * {@inheritdoc}
    */
-  protected function createSourceField(MediaBundleInterface $bundle) {
-    $field_storage = $this->entityTypeManager
+  protected function createSourceFieldStorage() {
+    return $this->entityTypeManager
       ->getStorage('field_storage_config')
       ->create([
         'entity_type' => 'media',
@@ -66,12 +67,17 @@
         // generic media type.
         'type' => 'string',
       ]);
+  }
 
+  /**
+   * {@inheritdoc}
+   */
+  protected function createSourceField(MediaBundleInterface $bundle) {
     /** @var \Drupal\field\FieldConfigInterface $field */
     return $this->entityTypeManager
       ->getStorage('field_config')
       ->create([
-        'field_storage' => $field_storage,
+        'field_storage' => $this->getSourceFieldStorage(),
         'bundle' => $bundle->id(),
       ]);
   }
diff -u b/tests/src/FunctionalJavascript/BundleCreationTest.php b/tests/src/FunctionalJavascript/BundleCreationTest.php
--- b/tests/src/FunctionalJavascript/BundleCreationTest.php
+++ b/tests/src/FunctionalJavascript/BundleCreationTest.php
@@ -1,6 +1,7 @@
 <?php
 
 namespace Drupal\Tests\media_entity\FunctionalJavascript;
+use Drupal\media_entity\Entity\MediaBundle;
 
 /**
  * Tests the media bundle creation.
@@ -41,9 +42,6 @@
     $page->selectFieldOption('Type provider', 'test_type');
     $this->assertSession()->assertWaitOnAjaxRequest();
 
-    // Make sure the checkbox for creating the source field is there and save.
-    $this->assertSession()
-      ->checkboxChecked('type_configuration[test_type][create_source_field]');
     $page->pressButton('Save media bundle');
 
     // Check whether the source field was correctly created.
@@ -64,17 +62,24 @@
   }
 
   /**
-   * Test creation of media bundle without default field.
+   * Test creation of media bundle, reusing an existing source field.
    */
-  public function testBundleCreationWithoutDefaultField() {
-    $label = 'Bundle without Default Field';
+  public function testBundleCreationReuseSourceField() {
+    // Create a new bundle, which should create a new field we can reuse.
+    MediaBundle::create([
+      'id' => 'pastafazoul',
+      'label' => 'Pastafazoul',
+      'type' => 'generic',
+    ])->save();
+
+    $label = 'Bundle reusing Default Field';
     $bundleMachineName = str_replace(' ', '_', strtolower($label));
 
     $this->drupalGet('admin/structure/media/add');
     $page = $this->getSession()->getPage();
 
     // Fill in a label to the bundle.
-    $page->fillField('label', 'Bundle without Default Field');
+    $page->fillField('label', $label);
 
     // Wait for machine name generation. Default: waitUntilVisible(), does not
     // work properly.
@@ -88,13 +93,15 @@
     $this->assertSession()->assertWaitOnAjaxRequest();
 
     // Un-check creation of default source field.
-    $page->uncheckField('type_configuration[test_type][create_source_field]');
+    $page->selectFieldOption('type_configuration[generic][source_field]', 'field_media_generic');
     $page->pressButton('Save media bundle');
 
     // Check that there are not fields created.
     $this->drupalGet("admin/structure/media/manage/{$bundleMachineName}/fields");
-    $this->assertSession()
-      ->elementsCount('xpath', '//*[text()="No fields are present yet."]', 1);
+    // The reused field should be present...
+    $this->assertSession()->pageTextContains('field_media_generic');
+    // ...not a new, unique one.
+    $this->assertSession()->pageTextNotContains('field_media_generic_1');
   }
 
 }