Did a German plane really land in Moscow in May 1941?

In the online version of Berliner Morgenpost Newspaper there is this paragraph:

Wenige Wochen vor dem Überfall tauschten beide Diktatoren Briefe aus, in denen sie sich ihrer gegenseitigen Bündnistreue versicherten. Hitler war an einem der raffiniertesten und erfolgreichsten Täuschungsmanöver des Krieges persönlich beteiligt. Am 15. Mai 1941 landete nach einem angeblich unautorisierten Flug in Moskau eine Ju-52 mit einem Brief Hitlers an Stalin. Das Flugzeug überwand alle Luftabwehr-Zonen, wurde in Moskau betankt und flog zurück.

Translation:

A few weeks before the attack the dictators exchanged letters, in which they assured each other, to hold on to their alliance. Hitler was personally involved in one of the most elaborate and successfull deception schemes. On May 15. 1941 after an allegedly unauthorized flight a Ju-52 landed in Moscow with a letter from Hitler to Stalin. The plane passed all air defense zones, was refueled in Moscow and returned.

This is during the time when Stalin received much information about the upcoming attack by the Germans but didn't believe it.

I don't know whether this is true, but there is some information about it in Russian Internet.

Russian Wikipedia article says

15 мая 1941 года немецкий Ju 52 вторгся в советское воздушное пространство и, пролетев по маршруту Белосток — Минск — Смоленск незамеченным, приземлился в Москве на центральном аэродроме около стадиона «Динамо».

Translation:

On May 15th, 1941 a German Ju-52 entered Soviet airspace and having flown the route Belostok - Minsk - Smolensk undetected landed in Moscow on the central aerodrome next to the "Dynamo" stadium.

The source for this statement is the book "Soviet Intelligence before June 22nd, 1941: Memoirs of Pavel Sudoplatov" (Советская разведка перед 22 июня 1941 (из воспоминаний Павла Судоплатова)).

The text related to Ju-52 incident claims that according to some sources the plane delivered a special letter from Hitler to Stalin (По некоторым данным, на нем доставили личное письмо Гитлера Сталину.).

Russian article says the following:

Внутри высших политических и военных кругов страны несанкционированный прилет в Москву Ю-52 особого всплеска эмоций не вызвал. Мерецков видел, что нарком Тимошенко, начальник Генштаба Жуков злополучный перелет Ю-52 не считали таким уж ЧП. Самолеты Германии часто нарушали границы Советского Союза, нередко садились на нашей территории, иногда даже группами. С октября 1939-го по май 1941 года таких нарушений было свыше пятисот, причем наибольшее число из них приходилось на первое полугодие 1941 года. Сбивать самолеты-нарушители категорически запрещалось. Наркомат иностранных дел СССР подавал официальные протесты Германии, на которые она не реагировала.

Translation:

There was no outburst of emotions inside highest political and military circles of the country after the incident. Meretskov saw that narkom Timoshenko, chief of the general staff Zhukov didn't regard the unfortunate flight of Ju-52 as a particularly grave emergency. German airplanes frequently violated the borders of the Soviet Union, frequently landed on our territory, sometimes even in groups. From October 1939 through May 1941 there have been over 500 such violations, most of which occured in the first half of 1941. It was categorically prohibited to shoot down these airplanes. Soviet foreign ministry sent official protests to Germany, to which the Germans did not react.

Another article claims that the route of that plane was this: Königsberg - Białystok - Minsk - Smolensk - Moscow.

That article says there are no traces of Hitler's leter to Stalin.

Однако никаких следов пресловутого «письма Гитлера к Сталину» до сих пор не обнаружено.

The same article says the Germans probably wanted to test the state of the Soviet counter-air defense and find out the optimal direction of attack in coming war.

вторая версия состоит в том, что немцы просто пытались исследовать состояние советского ПВО и разведать будущие направление главного удара вермахта.

Here are the details of how it happened that a German plane was not intercepted:

Наблюдатели приняли немецкую машину за рейсовый самолет ДС-3 (Ли-2) и никому не доложили о появление чужого аэроплана. Белостокский аэродром, получив информацию о пролете «Юнкерса», не поставил в известность командование близлежащих частей ПВО, поскольку еще с 9 мая, связь с силами противовоздушной обороны была прервана по техническим причинам. Последние, даже не думали восстанавливать связь, а вели переговоры с Белостокским аэродромом о том, кому надлежит восстанавливать коммуникации. В итоге, командование Западной зоны ПВО получило информацию о пролете Ю-52, только когда самолет немцев приземлился в Москве. Не знало о пролете «Юнкерса» и командование штаба 1 –ого корпуса ПВО города Москвы. Зато главное управление ВВС Красной Армии информацию о самовольно перелетевшем границу самолете получило, но вместо того чтобы задержать нарушителя, разрешило ему сесть на московском аэродроме и даже распорядилось службам ПВО обеспечить перелет.

Translation:

Observers mistook the German aircraft for a Soviet passenger place DC-2 (Li-2) and didn't tell anybody about it. The aedrdome in Białystok, did not notify nearby conter-air defense units because there was no communication with them since May 9th (for technical reasons). The latter didn't intend to repair the communications and instead negotiated with Białystok aerodrome on who is supposed to do this work. As a result, the Wester Zone Counter-Air Defense Command received information about the flight of Ju-52 only after the German airplane landed in Moscow. The command of the staff of 1st corpos of counter-air defense of Moscow also didn't know about it. But the chief command of the Red Army air force did receive information about the airplane that violated the Soviet airspace. Instead of intercepting it, they allowed them to land on the Moscow airfield and even asked the counter-air defense services to support the flight.

Li-2 was a Soviet version of the American DC-3 airplane.

I found what Sudoplatov actually writes in this book. The reference is in his memoirs. Here is my translation:

In May 1941, a German Ju-52 penetrated Soviet airspace and, unnoticed, successfully landed on the central airfield of Moscow, near Dynamo stadium. This created a stir in Kremlin and led to a wave of repressions among the military command: first people were fired, then arrested and the top commanders of the air force were executed.

He gives no further references or detail, but memoirs of Pavel Sudoplatov are generally trusted, and he really was near the center of events.

If you find the fact itself surprising, let me recall that in May 1987, a very similar case happened: a young German guy Mathias Rust flying from Germany, in a small private airplane penetrated all Soviet air defenses and suddenly landed on the Red Square, just next to the main entrance to Kremlin. Some top air force commanders were also dismissed, but no one was executed as far as I know.

Some air force commanders later claimed that they detected the plane but did not know how to react. (This was the time of perestroika, a radical improvement of Soviet-Western relations.) One can add that in May 1941 the relations of Soviet Union with Germany were also very friendly...

In 1987, Soviet ADF was supposed to be more advanced that it was in 1941. This accident made a great impression on the Soviets, and contributed to eventual end of the Soviet Union.

EDIT: Russian wikipedia has an article on this incident. They give a lot of references, all in Russian.